If you create a new custom observer based on a syscall observer, add it to the session, and re-start the session, one or more of the processes in the process group will not have a thread trace - just the proc name. See the screenshot...
Created attachment 1268 [details] Shot of the messed up trace Should be two lines - one for the process, and one for the thread. Notice how the threads box is also empty.
Might be more of a problem than I thought - when I exited Frysk this time, all the zsh processes from that screenshot were killed!