A few days ago I created a mod with a quest. The quest starts with a courier delivering a letter to the player. While testing this mod I came across a really strange glitch and can’t seem to resolve it. After I removed the mod (the .esp and bsa. files) the courier will still approach the player. He goes through the entire dialogue but doesn’t deliver anything. What could be triggering him to do this? I went through all my Skyrim folders looking for scripts (or copies of) and found nothing. I even removed all my back-up files. There is no sign of my mod in the Skyrim folders anywhere (that I can find), and yet the courier still appears.

I know this issue is related to my mod somehow. The character I'm testing this mod with should not be getting anymore courier delivered quests.

- Thanks

One possibility:

If you make a save while a script is running, that script gets retained in your save. If you then remove the mod, and restart the game from that save, the script is still there and active.

That may possibly be the #1 reason why people end up with borked games due to a mutually inconsistent set of scripts after removing a mod, and why removing mods is not officially supported, only adding is.

P.S.
That condition could under some circumstances even prevent you from updating the active script with a new version in a new version of your mod.