Male condoms that break in use do so mostly by a “blunt puncture” mechanism.
“METHODS: Over a period of 7 years, broken condoms returned to a supplier (SSL, Durex) via consumer complaints were examined to determine the cause of failure. Also, some consumers who reported breakage but did not return condoms were sent a questionnaire on the causes of breakage. Finally, theories proposed for the mechanism of breakage were investigated on a laboratory coital model. RESULTS: Nearly 1000 (n=972) returned condoms made from natural rubber and polyurethane were examined. Visible features on those that were broken, were classified. Evidence combined from examining returns, questionnaire responses and the coital model strongly suggests a single predominant mechanism of failure we named “blunt puncture,” where the tip of the thrusting male penis progressively stretches one part of the intact condom wall until it ultimately breaks.”
And the pièce de résistance, the “laboratory coital model” used to test condom breakage: