Currently, if a routine is scheduled in a nonexistent schedule group, there is a nonfatal warning at the time the CST is run. The reason that the warning is nonfatal is that sometimes it is desirable to schedule in a group which only exists if a certain thorn is activated, as the routine would not make sense without that thorn. However, mistyping the group name or not having a sufficiently up-to-date version of the thorn providing the group (see MoL_PseudoEvolution) will lead to the routine not being scheduled at all, with only a warning at CST time. This warning is very easy to miss.
I propose making the error a fatal error, and allowing the thorn writer to indicate in the schedule.ccl file that a function is scheduled only "optionally" in a given group, if that is the intention.