When using minecart hoppers (see picture for configuration) I noticed that the items from the top minecart were being pulled into the bottom minecart. It only happens when they're facing (to travel) East or West. It happens whether the minecarts are moving or not, on rails or not and even if the top minecart is on a slant. As long as they're diagonally one block apart like the picture shows, it'll happen.

ps. this also happens with minecarts with chests as the top minecart

pps. Given the correct angle, it will also do it to to the blocks that aren't parallel to it but diagonally too (second picture)