Ah, Brains, why do they not do the things that they should!

Edge cases are interesting. My recent working life has been all about 'and under what circumstances will this not go right; what do we do about it' and never managing to determine all of them, and thus having to deal. Including one rather 'entertaining' one were I got a complaint back about a review from a person who had left the group 5 years before, only to discover than they hadn't understood the difference between 'random' and 'balanced' allocations, and thus had done their allocation-to-group in strict rotation between the options.