Sometimes aquarium hobbyists encounter plant stowaways in their tanks. It's usually snails, which can be a bonus or pain in the butt depending on whether the tanks inhabitants can/will eat them fast enough to quell the potential snail-splosion.
I've also heard of hobbyists finding mystery fish fry in their tanks after adding new plants - fish often lay their eggs on plants which then hatch later in the new tank. If they survive (fry can be delicate) it's always fun to watch the fry grow up and see what they turn out to be.
Recently I discovered an entirely new kind of stowaway. As I was standing near the goldfish tank I saw what looked like a shrimp swim by, which is pretty strange in a shrimp-less tank. Upon closer inspection the shrimp turned out to be a tiny marbled crayfish, who was spotted again playing king of the castle on one of the moss balls! I hadn't added anything new to the tank for a month then, which means this little cray had done a great job of until it was no longer bite-sized to it's tank mates.
Marbled crayfish, or marmokrebs, while common in the aquarium hobby,