Yo yo-ing between cutting and bulking every other week. I'll start a bulk and then decide by the end of the week, I'm still not lean enough to bulk and switch to cutting the next week, then suddenly start to feel small and will bulk the next week. I did that for nearly a year without realizing the mental trap I was falling in to.
A second bad habit I have had in the past is going too extreme on my cuts because I am impatient with the dieting and want to get back to the bulking. By the end of the week my energy and mood starts crashing and I splurge on the weekends and have to restart the next Monday exactly where I was at before. Again I probably wasted 6 months doing something like this.
Final bad habit is routine inconsistency. Sort of in a similar sense of my bulking and cutting every other week, I have been guilty of always looking for that one perfect program. One week I am doing Mentzer's Heavy Duty, then the next an Arnold style high volume routine.