Xboard doesn't appear to handle pipes correctly when used with an ics helper such as openseal.
I noticed this problem on an ARM device using openseal. But investigation showed that I get the same problem on linux i686 with the ics helper as telnet.
It connects fine, but when you 'quit', the process hangs, xboard consuming 100% cpu. Typing enter, or ctrl-c closes it with a broken pipe error. But that's irritating, but also not always possible, as often I leave the computer to run an errand, and come back to xboard using 100% cpu for long periods (because FICS closes after an hour)