In the latest Minecraft release (and prior; this bug was experienced in 1.3), placing a block in a 1-by-1 hole (or seemingly next to 3 other blocks) will cause a massive lag spike. This lag spike is experienced by all players when playing on a server. It was difficult to determine what the extra memory was being allocated to based on the pie graph in the debug menu, though it seemed to be either "tick" or "render", and was certainly not "gameRenderer." It would almost seem to be the result of a memory leak, as the lag spikes get progressively more common and worse as more of these blocks are placed.

Note: While the screenshot denotes that my client is using Optifine, the lag spike was also replicated with a vanilla client on a vanilla server.