Falador medium pickpocket task without correct thieving level

What I did: Tried to pickpocket a Guard in Falador.

What I expected to happen: Nothing, as I had lvl-20 thieving.

What actually happened: Pick-pocketing failed successfully. However I completed the relevant task.

IDs of related NPCs/items: Guard, ID:3230

2009-era source (if relevant):

Screenshots or video:

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