If an assembler is combining different parts and you copy, then flip the assembler and try to feed it the same parts in the left and right outputs, it will get stuck....or if you reverse the left/right feeds while refactoring your layout...as long as the same 2 or 3 pairs are being fed in, it should be agnostic to which input is used. FYI, I do like that it seems to care about shape, but not color so you can copy/paste a white setup to yellow.