Occasional
From Hull AWE
Some younger writers have difficulty spelling this word.
It is quite simple. Occasional, like occasion (which seems to give less trouble), has two '-c-'s and only one '-s-' - not the other way round.
Occasion is one of the 117 mis-spellings listed as 'Common difficulties' in the section on 'Spelling' within 'Writing' in UEfAP.