12 Things an Expert Witness Should Never Say in Deposition
A field guide for attorneys who need to vet, prepare, and protect their expert witnesses
12 Things an Expert Witness Should Never Say in Deposition
A field guide for attorneys who need to vet, prepare, and protect their expert witnesses
Herbert Roberts, P.E. | forensicfailures.com
The Expert Witness Problem Attorneys Never Discuss Openly
You retained an expert with the right credentials. The CV checked out. The preliminary opinions looked solid. And then something went wrong in deposition — a phrase, a characterization, a moment where the engineer stopped being a technical analyst and became an advocate, a speculator, or a witness the judge had no choice but to limit.
It is not always the opposing expert who damages your case. Sometimes it is yours.
After three decades in engineering — including aviation R&D, failure analysis, and forensic consulting — I can tell you that the gap between a defensible expert and an excludable one is often a single sentence. What follows are the 12 statements that create that gap. Each one is a red flag you can listen for when vetting an opposing expert, and a guardrail you can install before your own expert walks into the deposition room.
The 12 Triggers — Organized by the Failure They Represent
Each entry includes the statement pattern, the legal or procedural red flag it raises, and what it signals about the expert's preparation, methodology, or understanding of their role.
Scope and Disclosure Failures
1. "My opinions extend a bit beyond my written report, but..."
Red Flag: Scope violation. FRCP 26(a)(2) requires complete disclosure of all trial opinions in the expert report. Testimony beyond that scope is excludable.
What It Signals: This expert either did not complete the report properly or is improvising under pressure. Either outcome weakens your case. File the motion in limine if this surfaces at deposition.
2. "The plaintiff is entitled to..." or "The defendant is liable for..."
Red Flag: Ultimate issue testimony. The expert has crossed from engineering analysis into legal conclusion — which is the court's domain, not the engineer's.
What It Signals: Opposing counsel will move to strike this testimony as an improper legal conclusion. Your own expert should know where engineering analysis ends and legal argument begins. If they do not, prepare for a Daubert challenge.
3. "Based on what the client told me, I concluded that..."
Red Flag: No independent investigation. The expert has adopted the retaining party's narrative without examining the physical evidence directly.
What It Signals: Expert opinions must rest on direct investigation, not secondhand accounts. An expert who has not personally examined the failed component, the accident site, or the relevant documentation has a foundation problem that opposing counsel will exploit in cross-examination.
Methodology and Daubert Vulnerabilities
4. "In my experience, this type of failure means..."
Red Flag: Speculation. No testable methodology behind the conclusion. This triggers Daubert's reliability requirement under FRE 702.
What It Signals: Experience alone does not meet the reliability standard. Federal courts have excluded engineers with decades of field experience who could not articulate a reproducible, peer-reviewed methodology underlying their opinions. Ask your expert: what is the method, not just the conclusion.
5. "I used my own proprietary analytical method to reach this conclusion."
Red Flag: Untested methodology. Novel methods that have not been peer-reviewed, lack known error rates, or fall outside generally accepted engineering practice will not survive Daubert.
What It Signals: The methodology must be transparent, reproducible, and grounded in published standards or recognized engineering practice. A method only the expert uses and only the expert validates is an exclusion waiting to happen.
6. "I focused on the most relevant evidence to my conclusion."
Red Flag: Cherry-picking. The expert has selectively presented evidence while ignoring data that contradicts or complicates the primary finding.
What It Signals: A qualified forensic engineer considers all relevant evidence — including evidence that does not support the desired conclusion — and discloses it. An expert who cannot explain the contradictory data is an expert opposing counsel will methodically dismantle.
Advocacy and Role Confusion
7. "What really happened here is that the company decided..."
Red Flag: Advocacy language. The expert has shifted from neutral technical analysis to arguing your case — which is your job, not theirs.
What It Signals: Judges have seen this pattern. An expert who sounds like an advocate loses credibility with the trier of fact. Your expert's job is to educate the jury about engineering principles, not to argue the verdict. If they cannot separate those functions, your case is at risk.
8. "The defendant clearly intended to cut corners to save money."
Red Flag: Intent and motive testimony. State of mind is outside the scope of engineering analysis and is improper expert testimony.
What It Signals: The engineer can testify that a specification was violated, that a lower-grade material was substituted, or that documented procedures were not followed. Why the party made that choice is for the factfinder. An expert who speculates about motive has handed opposing counsel a gift.
9. "I do not believe the witness is being truthful."
Red Flag: Credibility judgment. Assessing witness veracity is exclusively the province of the jury. This testimony will not survive objection.
What It Signals: The engineer can testify that physical evidence contradicts a witness account or that a described sequence of events is mechanically impossible. The conclusion about truthfulness belongs to the jury. An expert who crosses this line invites exclusion and signals to the court that they do not understand their role.
Credential and Boundary Violations
10. "As a structural engineer, I can also speak to the thermal dynamics..."
Red Flag: Out-of-discipline opinion. The expert is offering testimony outside their licensed area of competency.
What It Signals: PE licensure has defined boundaries. An expert who wanders outside their documented discipline — under pressure from counsel, in response to a deposition question, or through simple overconfidence — gives opposing counsel grounds for exclusion on that portion of the testimony.
11. "My CV reflects my extensive experience in this area." (When it does not.)
Red Flag: Credential misrepresentation. Exaggeration, omission, or implication regarding qualifications is grounds for exclusion and professional discipline.
What It Signals: Opposing counsel will verify the CV. Every claim about licensure, publications, prior testimony, and relevant experience will be checked. An expert whose credentials do not match their representations is an expert you cannot put on the stand.
12. "In my dual role as the company's engineer and your retained expert..."
Red Flag: Fact witness and expert witness conflict. Serving in both roles on the same matter creates disclosure and privilege issues that must be managed before they become trial problems.
What It Signals: The roles carry different obligations, different privileges, and different evidentiary standards. An expert who has not disclosed this conflict — or who does not understand why it matters — has created a procedural vulnerability. Identify and resolve this before deposition, not during it.
How to Use This List
Before retaining: Ask your candidate expert to walk through their methodology for a case similar to yours. Listen for the patterns above — experience substituting for method, advocacy language, scope creep beyond their discipline.
Before deposition: Review statements 1, 2, and 11 with your expert directly. Confirm the written report captures every opinion they intend to offer. Confirm credentials are accurately represented. Confirm they understand the line between engineering analysis and legal conclusion.
During opposing expert deposition: Run the 12 triggers as a listening framework. Any of these patterns, properly documented, supports a Daubert motion or a motion to strike specific testimony.
Download the Deposition Red Flag Checklist — the single-page field version of this framework, formatted for deposition prep and courtroom use.
This is Post 3 of 13 in The Forensic Engineer’s Field Manual. Read the full series at inventorsmindblog.com.
Herbert Roberts, PE | Licensed Professional Engineer | Six Sigma Black Belt
Forensic Engineering Consultant | 32 Years Aviation R&D | 62 Patents
inventorsmindblog.com