This morning, our trusty HP PhotoSmart C5180 stopped working in a very weird manner:
- According to the printer's display, all is correct 
- The printer can be discovered correctly (DNS-SD)
- The printer's web interface works flawlessly
- The printer accepts TCP connections to port 9100
However, data sent to port 9100 is not processed by the printer. Wireshark investigations showed the following:
- The printer is happily ACKing the packets it receives from the computer
- The printer's receive window goes down, starting at 17376 (which I already find rather small) in proportion to the data sent
- Power cycling any of the involved devices (computer, printer, switch) does not help
This happens from multiple computers (Macs, if you want to know), all of which were able to print yesterday.
My conclusion: The printer's TCP stack is not able to drain the buffer to the printing engine.
I guess that the printing engine has crashed, maybe due to low memory (as suggested by the tiny TCP window).
Does anyone have a guess at the cause of this surely weird problem or (even better) a remedy?