This only seems to happen to me when the application isn't starting up for some reason (crashed, error, OOM killer clobbers it). You can see if anything illuminating gets spit out to stderr onto the xterm by starting the application on the command line.

Maybe the memory usage spikes so fast and the OOM watchdog kills it so quickly that the CPU/Mem monitor just doesn't have a chance to update so it looks like nothing happened. Having said that, I have to open a fair number of applications before things start dying and things not working is usually accompanied by a message about running out of memory.