We record calls with Monitor (caller and callee separately) then run a script to combine two 'sides' in a one file. (there was such a need, tha'ts why we started doing it instead of MixMonitor)
So, we use 'Monitor' with 'm' option.
Sometimes I see a warning saying that running a script failed.
I can not understand why it happens from time to time. But in fact, the most of conversations are processed well.
http://doxygen.asterisk.org/trunk/d5/d60/monitor_8h.html here I found (with Ctrl-F in browser) 'Execute of', but as I'm not a programmer, it's not obvious to me what happens.