i think it's the knurling on the bar making it too thin. fairly sure it wasn't overtightened

 seen this happen to these bars a few times.. might be worth not doing stem bolts as tight though if you are doing them tight, who knows! all 22.2 bars seem to be a load of utter shit though, riding around on a set of bent dartmoors atm :/

 They definately weren't overtightened. Going to get a badaboom next which'll hopefully hold up

 Hope so