Having thought more about this, I think my offered solution about a goalkeeper changing places with an outfield player is incorrect, as both players would be booked in these circumstances.
The answer presumably relates to things a goalkeeper must do that others are not required to do (I don't think there is anything a goalkeeper can't do that others can).
The specific things relating to a goalkeeper are:
- He must not move forward from the goal line before a penalty kick is taken (he can move sideways)
- He must release a ball held in his hands within 6 seconds
- In the case of a penalty shootout, when his team are taking a penalty he must stand at the corner of the penalty area and the goal line
Infringing the above rules look to be non-bookable offences in general, but he could be booked for repeatedly flouting them.
Goalkeepers have to have shirts of a different colour from both teams and the ref, but I can't see him being booked for this - the ref should merely tell him to go and change his shirt.
There used also to be a rule about goalkeepers having long sleeves (so that the ref could distinguish the goalkeeper's arms when going up for a crossed ball), but that rule is now defunct (see Fabien Barthez).