In the scene where Wayne and Garth have just made up, they are sitting on stools at the counter inside Stan Nikita's Donuts. But when you look outside behind them, you see the building with the Stan Nikita's sign on the front, the building that they are supposedly in.
I doubt if this is a slip up. I believe that the director's wanted that there for a reason. I had always noticed it too when I watched the movie, but think of it this way. There are places with similar names for the store. Or even some places with the same. So that might have just been a different place owned by the same person.
It looks as if you guys have never been to Canada. Stan Mikita's is a spoof on Tim Horton's donuts (Horton played for the Maple Leafs), which are on nearly every corner is certain parts of the great white north. The joke is that you look out the window of one store and see another store (one on every corner) and I would not be shocked if that is really true somewhere in Canada. Since Wayne is a BlackHawks fan, they basically took an ex-Hawk (Mikita) and recreated the same type of place. Funny stuff, in the movie and in real life. You can find more commentary on this if you seach the internet...