Once in the very beginning (about two days into being vegan, I had been a vegetarian for about two or three weeks).
My mother can be very emotional (controlling and manipulative) sometimes... She cried when she realized I was not drinking milk, so I had some on my cereal to make her happy. That was the first and last time I did that.
She actually would not let me buy an alternative milk, so I just had dry cereal in the morning for two weeks. After that, I managed to make almond milk (believe it or not, she would not let me... she still thought I was drinking cow milk), but that only lasted a few days. Then, my dog seriously injured himself (broke his spine) and was going to be in recovery at the vets office for months and months... The day after we found this out, we went to Wal-Mart to buy him some toys, and I asked her again if I could buy soy milk... Everything was too much for her at the moment, so she just said yes.
I know I kind of used a bad situation to my advantage... but I don't regret it.
(if you are wondering, my lovely puppy did not make it... He ended up re-injuring his spine and had to be euthanized... I was right there when they put him to sleep, and it was something I can never forget. I swear he screamed... They tried to tell me it was a natural response, but I know it wasn't.)
Other than that, I have not fallen off. I have occasionally craved sea food (I've always loved sea food...), but I just deal with it and move on.