I'm using v15 of the arduino system and I'm also seeing this bug.
I'm connecting out from my wiznet shield to a local apache2 server and when I hard reset the boards, the first connect always works. then I can do some more connects but not reliably. when it gets in this state, it still responds to loop() events (I have an IR receiver that detects remote keypresses and that still works fine) but outbound connects just fail. then I reset the board and all is fine for the next one, etc.
it was not easy, though I tried freebsd and could not build the IDE. tried linux (gentoo) and could not get avr tools to work. tried ubuntu - yay, that worked. but I do my work on winblows (usb serial and all that) so I wanted to try building the ide from win source. had to edit the make.sh file a lot (almost all commented-out things had to be uncommented for cygwin). finally I got an ide that said v16, I loaded my ethernet web client source, built it and it works great now! no more 'lost packets' (lol).
ethernet now is a LOT better. before, well, it was useless.