I tried to edit an article to fix a typo.
The edit page told me "fix grammatical or spelling errors" and "correct minor mistakes" but then told me off for fixing too few characters and told me to avoid trivial edits.
Which is it?
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That message is made to prevent too minor edit to a question, that may cause it to be continuously "bumped" to the top. Many edits might also cause it to become community wiki, which will prevent the posters from gaining reputation.
When you edit a post to fix typos (which is perfectly fine) just keep an eye for more important edits to be made: formatting, phrasing, capitals and so on. That way you'll really improve the post and hopefully no further edits will be needed.