PPRE's wild Pokemon editor is kinda buggy. Sometimes it adds random garbage to the ends of script files on the map you're editing, which breaks the game. You can fix this by replacing the script .narc with a clean one from an unedited ROM, or by fixing the corrupted script yourself.
If you want to avoid this in future, setup an extra copy of your ROM, edit the wild Pokemon in the copy, then copy the edited encounter data narc to your actual ROM. This way only the scripts in the copy will be corrupted. You could also look for another tool that can edit encounter data, though I'm not sure if there is one for HGSS.