I donate on reflex. Especially if the fish is new, I will purposely rearrange my inventory so I can separate what I have to sell and what I have to donate. That way, I won't accidentally sell something that was not meant to be sold.
I do this also. I've only messed up once. I had never caught the firefly (let alone donated it) in my second town. I figured this out while I was on the island on the last day of the last month it was avaliable and when I had left for the island it was raining in town. I thought I was screwed. But luckly when I took the boat back home, it had stopped raining and it was before midnight. So I got my net out, caught the firefly and went straight to Blathers.