I'm sure this is not a widespread pet peeve except within the running community so many of you might not get worked up about this. At running (and other) races it is now the custom to provide all entrants with a race t-shirt. I am a big fan of this tradition since it has significantly increased the amount of clothing I have (although they usually don't have my size, but I'll discuss that another time). I especially love when they give you tech shirts or long-sleeved shirts, or better still, long-sleeved tech shirts. The Tufts Health Plan 10k has really nice ones and I recommend it.
But people... do not, I repeat, DO NOT wear the race shirt during the race. If you ran the race last year and you want to wear last year's shirt, that's fine, but you cannot wear the shirt for a race you haven't even completed yet. First of all, you typically receive the shirts on race morning. I have to wonder, all of you in the race shirt, did you forget to bring a shirt? Did you run out of the house topless and then count your blessings when the race director provided you with a shirt? I mean, really. Secondly, although some race directors will provide you a nice tech T-shirt, many of them are cotton, and cotton is not exactly the ideal racing material. Let's be honest here.
Lastly, and perhaps most importantly, it is practically the first rule of running that you never ever ever try something new on race day. I don't care if it's a dinky 10k. You never try something new on race day! NEVER! Not new socks, or new shoes or shorts or hydration schemes or paces or food or SHIRTS! Nothing new! Seriously! That is such a newbie mistake, and wearing the shirt just highlights your noobness.
Many runners agree with me, so you know it's not just me. You may change into the race shirt immediately after the race, however. That is totally acceptable. And perhaps preferable so you don't wear a stinky sweaty shirt around all afternoon. Gross.