Felipe Aguilar of Chile was assessed a four stroke penalty last week at the Turkish Open. The second round was not completed due to weather, and Aguilar switched out one club for another before resuming the round the next day. The relevant rule is 4-4, as follows:
"The player must not start a stipulated round with more than fourteen clubs. He is limited to the clubs thus selected for that round..."
If he had started with thirteen clubs, he would have been allowed to add one. However, swapping out one for another within a stipulated round? Verboten. The penalty is two strokes a hole for each of the first two holes he had the club in his bag.
It was a mistake of ignorance, not intent. Aguilar's fellow competitor, Magnus Carlsson, noticed it and asked Aguilar about it. Aguilar asked a rules official before he signed his card if he had committed a breach.