Hmmm... I played out to level 11 and didn't notice that. When I couldn't walk it was because the sides definitely didn't match up. A couple of times I caught myself as I thought they matched, but didn't. One of the last levels I played had multiple tiles that matched, yet were all useless, except for one. I liked that little trickery.

All in all, really creative - it's actually quite amazing how quickly and intuitively one can move the slides around too.