Event Editing, Help A Noob Plz~

Small detail but very important: make sure your hex editor is in Overwrite mode instead of Insert mode. The former edits only the highlighted byte, and as the highlighted selection moves that too is edited. The latter does not edit the highlighted byte but inserts a new value in it's place and move the entire rest of the file 1 byte back, making it 1 byte longer.

There should be a menu option, something to click, or a keyboard shortcut to change from Overwrite to Insert mode and back.

Source: someone who has screwed up a lot of roms via insertion.