I'm using the digital rules on the A-D-G websight. Have they not updated their digital version with their own errata?
>Red artefacts can only be placed if its counter is not already in play."
Do some artefacts appear on more than one card? It would be a good thing to mention, otherwise that sentence seems to be a non-sequitor.
Not only do most artifacts appear on more than one card, but cards used to start artifacts go into the discard pile, and there are several ways that cards in the discard deck may end up in a player's hand. Specifically:
1. When the draw pile is empty, the discard pile is shuffled and becomes the new draw pile.
2. Religious leaders allow draws from the discard pile during destiny actions.
3. The Vizier event allows one draw from the top of the discard pile.
Any of these might result in a discarded artifact card being in a players hand while the artifact is still in play.