I have some traces from a lab environment where probes from mobile stations caused congestion collapse. The active broadcast probes, on all channels, in an AP-rich environment caused the probe responses to use up all the tx-ops. It's ugly, ugly, ugly.
Unfortunately (or maybe fortunately?), the stations are all old kernels+drivers (Froyo Android, some kernel 2.6 devices, etc). But the problem is the basic operation of wifi APs.
(I wish I could share the raw pcaps, but I can't).