The Guitar Hero and Rock Band games created by Harmonix all have a strum limit. However, this was not deliberately put into the game, but it would have been easy to take out.
PAL is a video format that is used in some countries like Australia, New Zealand and all of Europe, however some countries, including America, Canada, Japan and Brazil, they use the NTSC video format. All NTSC PS2s and Xbox 360s run at 60Hz, which means the game is refreshed 60 times a second, all PAL PS2s run at 50Hz, which makes the game refresh 50 times a second. However, the PAL Xbox 360 is different, all games can run at 60Hz, but some games can be run at 50Hz.
The strum limit in these Harmonix games are based on the Hz the game is running at. It has been proven that games running at 50Hz have a higher strum limit, and so FC's of some songs are possible on that settings. Guitar Hero II was the first game in which there was a near impossible FC on 60Hz, and that is Trogdor. Guitar Hero II on the PAL PS2 is the easiest to get the FC on, there has only been one Trogdor FC on the Xbox 360 and that was by ShonSuperStar. All FC's on the PS2 have been FC'd on PAL, however witwix has FC'd the song and he wont say which format he FC'd it on as the user is American so he would be playing on NTSC mainly.
There is no real reason proven as why this is so, however richjohnny has suggested this theory and there are many others, tell us what you think.
"When the game refreshes it delays any input on to it until the game has finished refreshing, however sometimes you may strum while the game refreshes, and you may again when it's done refreshing. When doing this, the game thinks you strummed twice at once, because of this, it will only register one strum, thus making the game skip one of the strums you have made and then breaking your combo. It happens more often on 60Hz because the game refreshes more often on that, but the actual time it takes for the game to refresh is unknown, all we know is that on 60Hz, it refreshes 60 times a second, but how long is the refresh process? And how long between each refresh? We know its a matter of milliseconds but we don't know the exact amount."