This is a very clear cheat IMO: potato chips. I had some (exactly 1 oz) a few months back. I felt guilty as I ate them. (another sign of a cheat IMO). also, I think that if the item is known to throw many people or especially you out of ketosis, it's a cheat. my opinion.

There was a time a few weeks ago that I went out of town with a close friend whose father paid for everything (it was 'the family vacation') - including ALL my meals. And they eat at fancy places -- thankfully we were out west, so there was lots of meat choices). One night towards the end of the trip, we went out for dinner, and I couldn't find anything on the menu under $25 that was LC friendly, and I didn't want to stick it to them. Plus wine. So I ordered appetizers of lamb empanadas, thinking they would be ok. Well they were like a dumpling. What was I going to do? Not eat it? I could have, but there were 4 and I ate them. My friend knew how upset I was, though I don't think it would be visible to anyone else. It was obviously not enought to eat for dinner, so they also gave me a slice of the kid's pizza. So that was a cheat, but I don't personally count it that way, because it was a social issue. If it were my own family, I would give them to someone or take them home for someone and eat later on, or if my own dad were paying, I'd get the $25 entree. Anyway, I nipped it in the bud and didn't say I've blown it, I'm going to have XYZ. I ate that meal and didn't slip up again. It's just one of those "life goes on, have to make this a way of a lifetime" deals and these things happen. I didn't beat myself up too much, though I was worried I'd go out of ketosis.
Anyway, that was ~Aug 8, and since then I've lost about 5+ pounds, so I guess it was ok.

Sorry for such a long essay. Other than those 2 times, I've been cheat free. I also don't consider booze a cheat because although it slows down weight loss it doesn't stop ketosis, just suspends it. Though I am trying (so far unsuccessfully) to cut way back on that.

Good luck!