The commonest spelling mistakes are grammatical ones - confusing the homophones 'its' with 'it's' and 'your' with 'you're'. Other common mistakes involve unstressed syllables where 'e' and 'a' sound the same, particularly 'seperate' (instead of 'separate') and 'permenant' ( instead of 'permanent'). We also get double consonants wrong e.g. neccesary. The words I struggle with personally are 'sh' sounds spelt with a 't' or an 's'. I always hesitate over words like 'pretentious' .

These are simple things but a lot of people can't spell "than", especially North Americans. They write 'then' instead. Another annoying one is writing 'should of' instead of "should've". For me, the ie/ei thing is always tricky; 'receive', etc

words with "ie" or "ei" are tough: "friend", "ceiling", etc. For Americans in particular, "s" and "z" can be tough: is it "wise" or "wize"? Several place names are spelled very differently than they are pronounced: google the pronunciation of "Worcester". In my town, there is a road spelled "Kuyahaunegahoi" that everyone pronounces "cowgiskee".