What is the worst possible thing I can do?

The best way to ruin your case is very simple: LIE. If you lie or exaggerate (particularly under oath at a deposition or trial), you risk losing your entire case, even if the lie is meaningless.

Let's start with an example: A young male, client "A", is severely injured in a truck collision on the New York State Thruway. When the accident occurred, client "A" was a seat-belted passenger in a car and he could not be blamed in any way for the collision. Before his deposition, client "A" is asked whether he has ever been convicted of a crime and he answers, "Yes, armed robbery, but I did my time in the state penitentiary and I have been a model citizen ever since."  Honest and forthcoming? Guess again.

At trial, client "A" gives the same answer that he gave at his deposition, namely, he had one criminal conviction but has been squeaky clean since his one big mistake. On cross-examination, the defense lawyer has a field day by confronting client "A" with a list of criminal convictions that would have made John Gotti look like an angel.  Client "A" is proven a liar and his attorney hides under counsel table during cross-examination.

There is three certainties in life: DEATH, TAXES and DEFENSE LAWYERS WILL CATCH YOU IN A LIE. Do yourself a favor: if you consider lying or exaggerating, you shouldn't waste anyone's time by bringing a lawsuit.