I find that at least 70% of 16-19 year old students spell
college as
collage, even when the word is only a few feet/inches away for them to copy. About 40% regularly write using street/text language for formal purposes as in the following example:
dis b a gd plce to lrn da bilding trade an dat
Most older learners, even those in HE classes, regularly confuse affect/effect, advice/advise, personal/personnel, practise/practice, licence/license, its/it's, there/their, there/they're, your/your'e, principal/principle.
The most common error I have to tackle is confusing singular possessives with plural nouns - for example writing party's when they mean parties.