Key masking and key ghosting is a pretty common issue. Basically, if you hold down three keys, it can prevent a 4th one from registering. Or, it can sometimes cause a phantom key press, resulting in it appearing like a 4th key was pressed when it wasn't. This is because keyboards use a matrix system, and it varies from keyboard to keyboard.
Google "keyboard matrix" for more info.
Google "keyboard matrix" for more info.