These errors are idiosyncratic and often happen only once per error, perhaps when you are tired and your tongue tangles. The other kind of error is a violation of the prescriptive rules of a language, that is, the rules of Standard English. These are the rules you worked workbooks about in junior high school. You will be required to write almost totally in Standard English in college because the people of business and industry expect it of you. Learning how homey or casual your language can be on a given occasion is one of the things college is supposed to teach you.
Languages change randomly and constantly.
That is why dictionaries and handbooks go out of date. What well-educated middle class speakers will accept as appropriate language for public occasions changes over time. You can trust handbooks and dictionaries for about twenty years. Spelling is much more conservative than the spoken language. English spelling looks crazy only because its spelling has remained standard since about 1750 with very few changes, but English pronunciation has changed quite a bit.