Mistakes happen and people will be far more forgiving, if you own up to it the moment you notice.
There is simple etiquette that most golf tournaments and people will want you to do in order to redeem yourself.
The first is to inform another player of the breaking of the rule, making sure that those playing against you are aware of your mishap.
The second is to then take the 15th club and turn it upside down in your golf bag. This will signify that you have taken the club out of play and have no plans to use it.
Most people can accept that we are human, and we can sometimes miscount, though there will always be people who wish to complain.