MTU discovery problems in the Internet?

At least one of the MTU discovery failures that Jamshid observed is an
interesting artifact of the IP-over-ATM paradigm. It seems that if the two
ends of a VC don't agree on the MTU, MTU discovery can persistently fail in
the following way: a large packet with DF set will arrive at an ATM reassembly
buffer which is to small. Since the whole packet can not be delivered, the
next hop can not generate the proper ICMP message to adjust the MTU.

So, If you are using PVCs in the Internet, you must verify that the upstream
end generates the proper ICMP message for the down stream end.