When you build a tunnel with pressed ctrl, the tile after the tunnel entrance will have a double ribi (5 or 10). Therefore you can't build an slope on this tile e.g.
The patch will fix this behaviour.
You couldn't build a bend, either. Or, better, the bend was possible, but it become a three way intersection. Should this behaviour be fixed by this patch as well?
Yes, this would also be fixed. (I always deleted the first part of an tunnel in order to build a curve, too).