Assertion failed: what does it mean?

[shampie]

After just trying to remove road, I accidently selected more than just the road tile (see the red marked squares).
Top one is a building [square 1], center one [square 2] was a road crossing and bottom [square 3] has a bridge end.
The road got removed but this error popped up (see below).

(trying it again causes the same error) [Trying to remove a piece of road with selecting the building (squares 1 and 2) does NOT cause this error. Trying to remove the road draggin the 2nd and 3th square does cause this error.]

[benc]

When you get an "assertion failed" error, it usually means you've found a bug. Please submit it, along with a savegame, to http://bugs.openttd.org.