It does happen to me , more often then I would like. It does bother me but as long as I get back on track the next day I don't worry about too much.
If you take the perspective that out of 30 days of training you only slipped up 3 days that is 90% of the time you did it right. In the big picture it adds up to maybe a couple of ounces of muscle that you lost and that usually doesn't make a difference.
As long as you learn your lesson you won't suffer from it , if that goes on the whole week then you should re-evaluate what you are doing.
during the off season it will happen the odd day , especially on days that I had a late night and got up in the afternoon and then have to get up early the next day. Sometime our lives don't permitt a perfect diet all the time. During precontest I almost never let it happen but at the same time I have to sacrifice alot of living to do it.