If you have 2 hoppers on top of each other, both with for example arrows, and the bottom one not completely full, 2 items at a time will be taken from the top hopper. This would be because the bottom hopper sucks the items out of it and the top hopper puts its item in the bottom hopper. I think this was not intended, and shouldn't happen.
Linked issues
is duplicated by 13
8 more links
I too had this problem.