Here is another little nasty that is found sometimes in a planted tank.
They typically live deep in the substrate, or in the filter. The largest I have seen was nearly 4 inches long, it was munching on some mulm about 2 inches deep in the aquasoil, up against the glass. The smaller ones just look like red worm, and you have to look very close to see the bristles.
I believe they are harmless, since they seem to be a detrivore. However, I have not touched one to see if they sting like some marine varieties. There have certainly been no dead fish with a row of stings up their side.
IMO, this is the nastiest of the nasties you can find while working on your tank.