I can't really properly explain this one, so for once, I'm going to just list the steps to reproduce the bug:

1. Place (any) freestanding wire down
2. Place another freestanding wire next to it by right clicking the side of the first freestanding wire to place it
3. Repeat this process, but on the same, first freestanding wire placed.

Place sound is played, stack size is decreased, and blocks that can be replaced are replaced (at sideFacing + 2), but no wire is placed.

EDIT: Doesn't just happen when placing on the side of other wires. It happens when placing anywhere. Take a freestanding wire, look at the ground, place it, and then place it again and again and again all while looking at the same exact ground block you first looked at :P