You can place redstone dust against a redstone torch and/or a regular torch and it will be placed in the block space next to it. The bug happens when you attempt to do the same thing even after a redstone dust is already taking same block space. The game allows you to still place a block (redstone dust) even after there is a block (redstone dust) already there, resulting in a loss of redstone dust.

What I expected to happen was...:
The game would disallow/cancel my attempt to place the redstone dust because there is already a redstone dust occupying the block space.

What actually happened was...:
The game allowed me to place another redstone dust against the torch even after there was a redstone dust occupying the space next to the torch.

Steps to Reproduce:
On a flat surface
1. Place redstone torch or regular torch on the upper/top side of a block.
2. Place a redstone dust with your cross heir cursor on one of the sides of the torch. Redstone dust gets placed in the block space next to the torch.
3. Repeat step 2.