PC The Accidental creation of a major bug

[DingLyDellmeister]

Recently I've started modding Minecraft, learning how to as I go.
But, I seem to have accidently created 2 bugs;

1. When you click on an item in your inventory, it gets deleted.
2. When you place a block/use an item, the stack in your inventory gets decreased by 2, not 1.

What class(es) should I check to try to fix this?