The task involves looking at a picture and making a single button
press response from amongst 3 button choices. The response is
registered through a port device which is a MEG-safe button box (can't
use the PST serial response box in the MEG machine).

The error arises when a participant accidentally pushes two buttons.
When this happens, we get a mysterious '4' appearing in our data. A
'4' never corresponds to any of the stimuli (numbered 6 to 255) or any
of the response codes (numbered 6,7, or 8), so it is a bit strange.

Have any of you encountered a '4' as some sort of error code or arising accidentally from a doubled response?