Only wear a band t-shirt if you have heard literally every song they have ever done. Ever.
Here is an example of all of your previous, hard-earned cred going down the drain, submitted by Andy:
Black Flag T-Shirt
I was at a gig with my friend and he was wearing a Black Flag T-Shirt with the famous logo on it. As we waited for the headline band the famous Black Flag song ‘Damaged II’ started to play over the PA. After a couple of minutes he asked me if I knew who was responsible for the song because it sounded pretty good.