Just because you haven't had any carbs doesn't mean you automatically get into ketosis. There's only about a thousand other factors involved.

If ketosis is what you're after, your liver has to have enough fat to work with - whether from the food you eat or your own, so to speak. I've noticed that as my bf% lowered, it's become harder and harder to get into ketosis - now it takes me at least three days, running every day, eating 0 carbs and only moderate protein (ca. 0.75 g/lb).

From reading on this and experimenting on myself, it seems that many more things can block your entry into ketosis than kick you out once you're already there. For example, Zeus pointed out citric acid. It's thought to cause problems because it serves as an intermediate in glucose metabolism (TCA cycle for you biochem nerds like me). More citric acid - more active TCA cycle - more amino acids converted to glucose - body doesn't need to turn on the ketogenic pathway. Indeed, skipping the diet coke early in the week helps me get into ketosis faster. But once I'm already there it doesn't bother me - that makes sense if the ketogenic pathway is already up and running. Same applies to trace carbs in what you're eating, and the fat/protein ratio - once ketotic, protein intake can go up a little bit.

But anyways, note that I said "if ketosis is what you're after". Can you lose weight and not be in ketosis? Sure you can...