I realized that I'm still a little confused about resolving effects that happen at the same time.
In
this post, which was thumbed by Donald, it's said that if my Prince plays my Throne Room, and I choose to TR a Guide, then after playing and resolving it twice, I can call it from the Tavern mat afterwards. So the Guide was placed on the Tavern mat in the beginning of my turn, and since it's still the beginning of my turn, I can call it right away. It was of course not on the Tavern mat when I started resolving effects that happen in the beginning of my turn.
A comparison is made with the age-old Moat and Secret Chamber. The Moat was not in my hand when I started resolving effects that happen when the other player played an Attack. The comparison seems reasonable.
However, in some ways this seems to contradict the rule that says that when several effects happen at the same time, you resolve all of them even if the conditions changed. See
this thread. I buy a Mandarin with Royal Seal in play. I resolve Mandarin's when-gain first, putting all Treasures on top of my deck. I can then resolve Royal Seal's when-gain even though it's no longer in play. (Procession playing a Band of Misfits-as-Fortress also seems to be a version of this.)
So if several effects trigger at a certain time, I even resolve an effect that had the triggering condition changed before I could resolve it. But I also resolve an effect that had the triggering condition change the other way: the condition didn't allow resolving it at first, but changed in the meantime. It's not exactly a contradiction I guess, but it's damn close. Or am I thinking about this wrong?