In the latest snapshot, firing arrows at a corner where two blocks meet in specific locations can cause the arrow to phase through. This is probably caused by the arrow thinking there is a gap in the blocks to go through.
EDIT: forgot to add, haven't tested fully in 1.8.8 yet (since there are no tipped arrows in 1.8).
Is this still an issue in the current Minecraft Snapshot 15w44b or later? If so, please update the affected versions in order to best aid Mojang ensuring bugs are still valid in the latest releases/pre-releases.