Corrected entry: In the opening credits, when all the boys (led by Arnold) are walking down the alleyway, on our right, Harold looks like he's about to walk straight into a bin. After the girls have done their strut, we cut back to the boys - and there is no bin behind Harold.
Corrected entry: When the people in the buildings yell "Arnold", Sid and Eugene have switched places.
Correction: The way they were when we saw them walking on the street yes. But when this happens the boys are in the alley. Since we haven't seen Sid or Eugene in the alley previously there's no mistake in where they're standing.
Correction: Actually this was Ruth that happened with. Anyways when we see the girls in the alley again (where the bin last appeared) the bin is indeed behind Ruth. You can see it where the back part of the alley is lighted.