An unusual situation arose in a session I was GMing last night.
One of the PCs (a conjurer wizard) was in the middle of casting summon monster IV, a spell with a 1-round casting time. The BBEG's turn in the initiative order was immediately before this PC's turn. I had the BBEG cast dimension door in the round after the PC wizard began casting the spell (but before it was completed) and placed said BBEG within 10 feet of the PC wizard (BBEG was large sized and had 10-ft. reach). BBEG's turn is now over and the PC wizard's turn is next.
I ruled that (in general) as soon as BBEG's turn was over, he gets an attack of opportunity on the wizard who is still in the process of casting his 1-round spell (i.e. still engaged in performing a distracting act). This was also a crucial moment in the encounter, as the AoO by the BBEG would be an almost guaranteed hit and an almost guaranteed disruption of the spell. I felt my judgment was 100% correct except for the unusual circumstance that the PC wizard's turn began immediately after BBEG's turn ended, and spells with a 1-round casting time complete "immediately before the beginning of the caster's turn."
The explanation I gave to my players was that I felt completely confident that the BBEG would definitely get an AoO on the wizard in any circumstance other than the specific circumstance of the wizard's initiative being immediately after the BBEG's. Had there been even one player between the BBEG and the wizard, I believe an AoO was in order. In other words, I felt there was no clearly defined amount of time that existed between the end of BBEG's turn and "immediately before the beginning" of the PC wizard's turn. Therefore, I ultimately decided not to take the AoO.
So, am I right or wrong in general (as far as taking an AoO in the general case of placing an attacker in melee threatening reach of a caster currently engaged in casting a 1-round spell), and did I make the correct decision in this specific instance? Thank you in advance for your comments and input.
