When I moved home from my old university, I had a betta. I was so excited about being home, and so tired from the drive and finals that I left everything in the car and was planning to unpack it the next day. I forgot about my betta.
Oh, did I mention it was December? I froze my fish, and then tried to be brilliant in saving it and tried to thaw it out. Needless to say, it didn't work.