Received the following error one time after many hundreds of hours playing with this mod:

The mod Math combinator (1.1.4) caused a non-recoverable error.
Please report this error to the mod author.

Error while running event MathCoProcessor::on_tick (ID 0)
Index out of bounds: 21
stack traceback:
[C]: in function 'newindex'
__MathCoProcessor/control.lua:324: in function <MathCoProcessor/control.lua:303>

Follow up - This seems to happen consistently when I add efficiency module to the green circuit being monitored by a math combinator set to filter. No other item seems to cause it. This is a K2/SE run, so I think the modules may be different entities from the vanilla modules.

Edit - the specific item had nothing to do with it, was just coincidence.

Issue solved. Math combinator entity only has 20 output slots, so when there were more than 20 output signals, it crashed.
Needed to add entity.item_slot_count = 100 to entity.lua (note that 100 was arbitrary and can be any number really)
Also added a check to the actual execution functions in control.lua so that if there were more than 100 outputs, it would just truncate to 20.