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)
• Forget to clear the camera lens. Sometime we touch it preparing the launch
• 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.
• Leave something behind. Like the balloon. Or the payload.
• Let the chase car battery go flat. If the battery is old it might not cope with 1-2 hours of preparation time whilst running the tracking kit.
• Assume that yesterdays predicted path will be OK for today, so it takes a path it really shouldn't. Winds change, so check right up to the launch time.
• Overfill your foil balloons, so they burst almost immediately.

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

• Make a checklist but don't actually follow it. Or leave the list behind.