I would say assassins were on my list of (many) mistakes over time....I bought a couple to deal with ramshorns and they multiplied like rabbits in my tank. Now I have an assassin snail problem, and I'm still picking out those suckers whenever I find them. They killed my lovely adult mystery snail back when there was a ton of them in my tank. Now I only find them occasionally, thankfully and all my other snails are fine...
What else, hmmm. Buying a fish without doing a ton of research first. Emperor tetras that I bought bullied everything in my tank, ate the rili shrimp I tried to add within a day of them being in the tank....ugh. Also, the very first fish I bought for my tank after doing a fishless cycle were beautiful Gertrude's spotted blue eyes. I had them for about 6 months, added another couple fish from a different LFS
and they were carrying some kind of disease....lost all my beautiful rainbows (and the new fish of course) to a fluffy white spot disease....sigh. Now I'm more careful about what livestock looks like when I buy and where it comes from.
Oh, and thinking a rescape of my tank would be easy. Ha!