The unwritten rule of music (mostly but not exclusively metal) that states that only Iron Maiden
are allowed to wear their own band shirts on stage, by virtue of being Iron Maiden. It has been broken numerous times. The worst offenders are probably Amon Amarth
, who wear their own merchandise in every photo and every video in which they aren't shirtless.
The reason for the rule is the same reason why it's generally gauche to wear a band shirt for the band you're going to see: band shirts are generally a way to broadcast your tastes and in so doing support a band that people might not have heard of.
Watain broke the Iron Maiden rule by painting their own emblem on their bare chests.