The bug
Torches and redstone torches can be placed on the horizontal sides of redstone blocks and observers despite the fact that these sides are apparently not considered solid since you cannot place for example buttons or levers on them.
MC-117470
Torches and redstone torches can be placed on the side of redstone blocks and observers
Resolved
Works As Intended
0
Confirmed
observer, placement, redstone_block, redstone_torch, side, torch
Minecraft 1.12 Pre-Release 2
Retrieved
I just checked release 1.8.9, torches were able to be placed on the sides of redstone blocks back then and were probably always able to be as I seem to remember a lot of people using that behavior to start redstone contraptions that required a short pulse.
The ability to place torches on the sides of redstone blocks is very likely an intended feature. I can't necessarily say the same about observers although this behavior did appear in the 1.11 releases, but if anything, it's stranger that buttons and levers can't be attached to the sides and undersides of redstone blocks, observers and pistons than it is that torches can be placed on the sides of the first two.