Seems that when you place a sign on the ground (the one with a base structure)
and then change the possition of the track with the copy/paste modus
You'll get a bug like this


there is no way to delete this
perhaps if you find this out soon enough you can use the undo function
but if you save and quit .. you're doomed

so be aware

Reported fixed: http://forum.maniaplanet.com/viewtopic.php?f=259&t=4996