Having a small clam walk away from the rock you placed it on, forcing you to move rocks to track it down before it dies! (Well, ditto for frags, but frags don't often walk away!)
Having anemones walk round your tank killing a path behind it while is searches for the "ultimate anemone home" which ends up being on a pump and it gets itself mangled!
Buying $60 worth of berghia nudibranchs ... and finding small blobs in the container you were quarantining them in!
Buying some fish that you love, quarantining them for a few days, then finding the dreaded "dead fish stuck to pump input".
Dropping a plastic knife into your tank where it settles between some rocks at the bottom of your "rock wall" and having to leave it there for several years... then having people look at your tank and say "What's that? It almost looks like a plastic knife?!"
V