This also happens on servers.

When you power a downwards facing dropper, its item will go anywhere.

Steps to reproduce:

• Place a downwards facing dropper in the middle of the air
• (optional) place blocks on the side of it, not underneath nor above.
• Power the dropper
• The items will fly in a random direction, and never will they go straight down. They will fly through the blocks beside the dropper too.

The dropper in the picture is facing downwards, I powered it around 10 times. Most items ended up above it...