Young children have trouble with irregular past participles. Once they get the idea of regular past simple forms into their heads, they try to make everything regular. So they go through a short phase of saying 'I goed' and 'I throwed' and so on. As for adults, it's definitely punctuation. You only have to browse these pages on italki to see dozens of punctuation errors made by native speakers. Typical errors are confusing 'your' and 'you're' and 'its' and 'it's'. More than once, I've even come across professional teachers here who make these mistakes, and even mis-correct notebook entries. There are certain common spelling errors which adult native speakers frequently make. Definate (instead of definite) and seperate (instead of separate) are among the worst offenders.

According to my mother, when I was learning to talk. I would say to her, "You do it! You do it!" So, she would start to do something, and I would get frantic and say "No! No! YOU do it! YOU do it!" Eventually she realized that of course, she used the pronoun "you" to refer to me, and "me" to refer to herself. So I believed that the word "you" meant "Danny" and "me" meant "Danny's mother." What I was trying to say was "let ME do it!"

Irregular verbs, long words and maybe prepositions for kids? Obscure words and occasional grammar like present subjunctive (not tricky; just unexpected) can throw adult speakers.

Punctuation.

Based on the time spent in school and on the need for spell-checkers in a word processor, I'd say it's spelling.