Here's a (non exhaustive) list of things not to do. These mistakes are very easy to make in the rather tense period when preparing to launch.

• Forget to clear the camera memory card (thus the camera stops recording part-way through the flight)
• Accidentally stop the camera or camcorder from recording before you close up the payload box
• Measure the neck lift wrongly. In windy conditions it's easy to think you have more lift than you do. Only measure when the wind is gentle, or do it indoors, otherwise you may find your flight ascends too slowly and thus travels too far, thus possibly ending up in the North Sea (cough).
• Allow the aerial to get bent over and forget to straighten it. It makes the telemetry signal much weaker than it should be.

All the above easily avoidable by remembering this one, big mistake:

• Make a checklist but don't actually follow it.