When I was a teenager one boob fell out of my swimsuite after splash landing on a waterslide in a swimming pool. Didn't realise at all till I saw my best mates brother, who I had a massive crush on at the time laughing at me. Thought I would never get over that.
You are not the only one darling! I feel the power of laughing about these embarrassing moment and getting them off our chest (so to speak) on this post.
Also - Isn't it strange that you never know what your boobs are up to if you don't keep your eye on them!