What really irks me is when 'professionals' fail on simple grammar rules and the real bugbear is when a therapist of whatever kind writes things like "I practice in a holistic way" instead of "I practise in a holistic way". At school I was taught that in words like practice - practise, advice - advise, licence - license - the noun of the pair was written with a c and the verb of the pair was written with an s - and the easy way to learn this was to think that c+ n, and s + v and that c came before s in the alphabet, and n came before v.
My English grammar mistress who of course must now be long-since dead would be delighted to know that I'm passing her rules on all those years later!