Some of mine in no particular order:

• Comma splices.
• Using apostrophes to make abbreviations plural. It’s UFOs, not UFO’s. This goes for decades, too. It’s 1920s, not 1920’s.
• Putting punctuation in the wrong place when parentheticals are involved (like this.) (Or like this).
• Same for quotations. Programmers in particular seem averse to putting punctuation on the inside where it usually belongs.
• Mixing up insure, ensure, and assure.
• Using ‘that’ where ‘who’ is more appropriate. For example, “People that don’t use their blinkers are annoying.”