Path MTU Discovery (PMTUD) is widely believed to be unreliable because of firewalls that discard ICMP “Packet Too Big”messages. This paper measures PMTUD behaviour for 50,000 popular websites and finds the failure rate in IPv4 is much less than previous studies. We measure the overall failure rate between 5% and 18%, depending on the MTU of the constraining link. We explore methods webserver operators are using to reduce their dependence on PMTUD, and find 11% limit themselves to sending packets no larger than 1380 bytes. We identify a number of common behaviours that seem to be software bugs rather than filtering by firewalls. If these are corrected PMTUD failures could be reduced by 63%. We further find the IPv6 failure rate is less than the IPv4 rate even with more scope for failure in IPv6.