I was a vegetarian for years, then ate meat, then went vegan (or followed a vegan diet, or a dietary vegan, or whatever - I know IS gets upset when you phrase it wrongly, but I'm sure you understand what I mean) and despite daily sublinguals and all kinds of fortified foods, over 3 years my b12 levels sunk so low I was showing physical symptoms of deficiency. We tried to "treat" it for two years. Apparently there are people who just can't get what they need through fortification. After that my only choice was to experiment with b12 injections every two weeks, which I don't see as a solution, or try to work in some alternatives, so I cycle in some animal protein every now and again. It's a better option for me than permanent neurological damage.
In my case I didn't "fall off the wagon" but made a deliberate, calculated choice. I don't see my case as apostasy; I think it depends on your motives in the first place before you can call it that.