never saw one happen - but have seen three just after the fact -

one an old low grade American double with fluid steel - the shooter admitted to snow in the barrel - had not checked it after going thru a snowy hedgerow

one at an early Vintagers - the small bore damascus was taken out for a test shoot and a barrel split- then someone measured it and it was paper thin

third was at a gunsmiths, twist barrel split on top 2/3rd down and again- paper thin after being reamed

these last two were thin enough they have been damaged by a strong grip or setting them in the rack too hard.