It's a major deal with race organisers Marky:
1. They've refused people entry into the event based on capacity, and now there's others coming in the back door
2. If an athlete collapses out on the course (and this happens at least once in pretty much every big race), the medical support team will have all the wrong info from the number
3. If you give the pass to a faster athlete (or from a woman to a man), it can totally screw up the AG and the team prizes
4. The entry is "Carmona, Race number 1234" but it's not Carmona racing at all - so the results will be attributed to the wrong person - like getting someone else to sit your leaving cert. People have used this trick to get a BQ in the past and there was uproar.
Even in local races, it's a big no-no, and like the example C. gives there, they really go after you. One of our running circle took an unofficial number swap at the Ballycotton 10 a few years ago - he was banned from the event for a year, and was only let back into local racing after a grovelling apology.