The bug
If you're holding something in your main hand, then walk up to a block that doesn't have a right click function (Ex: A Fence) and hold right click, then your off hand will start derping out. It looks like its trying to break the block.
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Comments 5
This may be intended. For whatever reason, fences have an interact action when you have no item in your hand. But if you have an item in your hand and that item can't be used, then the fence doesn't take any action either... which means that it moves on to the off-hand. And the off-hand, when empty, can also take that same interact action that you'd have with the main hand.
If you have items in both the main and off hand, then no swing happens.
Can confirm for MC 1.12.1.