Unlike in previous versions of Minecraft (1.5.2 etc), when playing on servers that you have bad lag, running can take upto 2 seconds (or more, depending on ping) to actually 'start running'. This becomes quite annoying for things like parkour, or PvP maps.

This also contributes towards players getting slingshotted through the air (in some cases, 20 blocks or more). This can occur when a server is unresponsive, a player tries to begin to run, and then gets teleported to a alternate dimension.