The latest installment of Call of Duty - Modern Warfare 2 - has received many positive reviews since its release (e.g., on Metacritic the PC version of the game boast an average rating of 8.9/10 from users). However, the good reception of the title from Activision Blizzard does not mean that it does not have any bugs. Quite the contrary.
As is usually the case with fresh games, they are prone to sprouting many defects, overlooked during testing. The latest example of this in MW2 is a bug that many players experienced during online gameplay in various competitive modes.
The aforementioned flaw allowed for the use of wallhack, a cheat that involves seeing other users through walls. This gives players a huge advantage, because they always know where the opponent is hiding.
How specifically did this manifest in MW2? It was related to the so-called ping function, which is usually used to notify team members of certain landmarks on the map. When a player died and used this system on the death screen to mark the opponent's comrades, the marker placed on the enemy did not disappear for the rest of the match (via Eurogamer).
As a result, many users began complain online about the unfair advantage their rivals have. Below you can see a video showing how this quasi-wallhack works.
Due to the huge buzz among the community, Infinity Ward decided to disable the ping function until they find a way to fix the game.. At the time of writing this news, no official statement on the matter has been issued, but given the developers' quick response, we can expect an imminent resolution to the problem.