I'm probably the most paranoid about goofing up on this game. I've got printed out lists of everything. Well, I took what I thought was my last fish to the museum and Blathers said nothing. I did another check by reading each display. Apparently I had caught two (easy) fish but had not taken to museum. OMG. Luckily they were year-round ones, but just because it's showing in your encyclopedia doesn't mean you've donated it.