If you place a torch, button, rail or lever (I'm sure other similar blocks as well, but not stairs or glass panes.) Then attempt to place an anvil in the same space (by placing it against an adjacent block) it will overwrite the original block.

This happens in survival or creative, single player and multiplayer. Does not seem to be client side, and does not work with any blocks I tested other than anvils (tried netherbrick, sand and dragon eggs).