My last true accident was when I was about fourteen or so. As a young kid, whenever I had dreams involving a pool or otherwise me swimming, that was a trigger for my body to start peeing but I caught onto it early on and thought I fully fixed it (it never happened again except this once). By the time I was fourteen, I was well and truly potty trained but one inexplicable night after having this dream I completely ignored the conditioning I'd instilled within myself and wet the bed and I was really embarrassed when I woke up. I told my mum and she said it was totally fine, it can happen to anyone at any time. It has barely crossed my mind since.
Beyond that, there's only been one time that could have been classified as kind of an accident because I didn't realise the full extent of the situation. One day when I was eighteen and starting to deeply explore my abdl side for the first time, I'd made myself a coffee and sat down at my desk to play some Overwatch. Naturally I really needed to poop and I knew I would need to, but when I started it almost felt like I couldn't stop, I messed so much it went all the way up the back of my diaper. I remember thinking to myself "oh my god this is so much, I've never messed this much before!" and having to wait until the end of the match to clean myself up. It was a lot of fun but also a big clean up, ehehe