The worse mistake is to lie. A deposition is a statement under oath. A lie under oath can ruin your entire case.
If you don't know the weight of the box you lifted, then don't guess. If you don't know the height of the ridge you tripped over don't guess. If your guess can be easily disproven, then it could taint your entire claim
Don't fail to Prepare!
You may given prior statements. You may have prior medical history. You need to review everything that may be relevant to your claim. Inconsistency between your current testimony and prior statements can ruin your case.
Listen to the question!
You need only answer the question that is asked. But to answer, you have to listen to the question. If you don't understand it, ask for the question to be repeated.
Review prior discovery of both sides!
There may be answers to interrogatories by both sides. There may be other depositions. Review them.
Don't volunteer anything!
A deposition is not the time to make speeches. You simply answer the questions. Anything you volunteer could ruin your case.