Sennheiser. Seriously, their firmware updates are a complete pain in the ass.
Updating the receivers HAS to be done through their Wireless System Manager. That's all fine and good. But to update the microphones, you have to place them in front of the receiver so the firmware can be flashed over IR.
So far the update crippled one receiver (which we fixed despite their customer support telling us it "needed a new board") along with about 4 microphones...
Oh yeah, and to use WSM, you pretty much HAVE to let them use DHCP, because while you can set the IP manually, you CAN'T change the subnet...and their hardcoded subnet is not exactly a standard string, either.