Skipping your warm-up – not warming up before a workout significantly increases your risk for injury, even if you are only planning a short workout.
Waiting for equipment – your rest period between sets should be kept short; waiting for equipment could give you too long of a cool-down, making your workout less effective. Always have a backup plan in mind incase the equipment you are heading to is occupied.
Not having a plan – to stay on task and have an effective workout, always have a workout plan ahead of time so you are not wasting time planning your next move.
Having the wrong plan – performing your cardio first could fatigue you too much to have an effective (and safe) lifting session. Complete your lifting first then head to cardio.
Performing the same old routine – completing the same exact workout every day will not only get boring, but will also stall your results. Mix it up every couple of weeks.