Most Cannizzaro mistakes come down to one thing: forgetting to check for alpha-hydrogens before picking a mechanism.
Mistake 1: applying Cannizzaro to an aldehyde that has alpha-hydrogens
If the aldehyde has even one alpha-hydrogen, it will follow the aldol pathway under basic conditions instead — Cannizzaro only applies to aldehydes like , , and that have none.
Mistake 2: writing the wrong pair of products
Cannizzaro is a disproportionation — one molecule is reduced, one is oxidized. Half the marks lost here come from writing only the alcohol or only the salt, not both.