Hijacking Ctrl+F or Ctrl+K are my two top peeves lately.
Don't fuck with the standard key combos.
Stop surprising users by taking away functionality from other applications.
In Google Sheets on the Mac recently, Cmd-F for searching within the spreadsheet has stopped working—and while it's true that the way it worked was by hijacking that system shortcut, it didn't actually replace it with anything. So the only way to search was to go to the Sheets menu bar and find it in the menu.