Easy:
1. Never assume anything.
Once went into a private site where I had been with an instructor on a previous occasion. I had thought it was OK to use it as it was OK to use the other sites that we used for practice. Well, the owner was p*ssed. I had assumed we had blanket permission to use this site for training when evidently we did not.
On another occasion I got into trouble at a well known airfield by using the same approach that I had used on my last visit three weeks earlier. They had in fact just moved helicopter ops to the other side of the field. I had assumed that it was the same as before and the guy I had phoned for a briefing immediately before the flight had presumably assumed that he didn't need to tell me about the new procedure.
2. If you are not sure what just happened, shut down (or land) immediately and check it out.
Whirly's experience should be enough to convince us all of that. I have twice found (thankfully minor) mechanical problems before lifting: one occasion was a vibration that I didn't expect, the other I only imagined that I heard something unusual but shut down anyway and yes, there was something wrong.