Holding your breath
This is the most common error and the easiest to catch. If your breath stops the moment you squeeze, you are bracing rather than training, and the extra pressure pushes down against the lift you are trying to create. Cleveland Clinic's men's guide flags breath-holding directly, alongside overuse of the abdominal, thigh, and buttock muscles.
The fix is simple: shorten the hold until you can keep breathing normally through the whole thing. Three seconds with a steady breath beats ten seconds with a stopped one.