When I got my very first Tamagotchi (I was eight), I didn't fully understand how it worked. It was a V2, and when I first started it up, I was so impatient that I just set the clock to a random time and started playing. Of course, once it evolved into the child stage, it immediately went to sleep because I had set the clock to like 2AM military time. Wanting to play with my Tamagotchi and getting frustrated with it not waking up (and being too dumb of a kid to even think of changing the clock), I'd reset it over and over again. The poor thing had to have been reset over ten times within the span of my first day caring for it. I think I also reset it a couple times after that just because I was bored, or got a character I didn't like. Somehow, it eventually lived long enough to evolve into a Masktchi named Zelda, who had a son named Mat (no, not Matt - just Mat). I still consider her to be my very "first" Tamagotchi pet, because she was the one who actually survived all my childish ignorance.
Another one... this isn't necessarily something I have personal guilt over, but when I was younger, I bought my sister a Tamagotchi V4 with some money my aunt had given me. I also got one for myself, and we had a blast helping each other take care of them, connecting them, and even frantically searching for them between couch cushions. At some point during the winter, my sister's Tamagotchi went missing and no one could figure out where it had gone. Eventually, we forgot about it... until spring came. Out in the backyard, under a swing we had hanging from a large tree, was her Tamagotchi. It was in near-perfect condition - aside from a few dirt smudges, the snow had made the perfect, frozen shield. Once it all had melted, we found it out in the grass, almost like an Easter egg. We brought it in, wiped it off, and replaced the battery, but nothing happened. Between the cold and the wetness of the snow, it was done for. I guess I mostly feel bad about this one because I'd love to have that working Tama in my collection today. Even though it wasn't my fault, it was such a pretty-looking shell, plus it held a lot of memories for me and my sister, so it would've been cool to still have it. RIP, adorable pink-and-green Tamagotchi V4...