Realizing that you and your group has been using a 3.5 rule incorrectly for 5 years… the week before you switch over to 4th edition.
Please tell me that my group isn't the only one that was under the impression that if a spellcaster took damage, they had to make a concentration check on their next turn in order to cast? What's scary is that I've been a part of 3 groups and every one worked that way. So, maybe it is the correct rule? But when we went digging this week, all we could find was concentration checks for if you take damage during a 1 round or longer spell, take damage as an AoO when casting a spell, or are taking continuous damage when you cast a spell. We can't find ANYTHING on making concentration checks after you take damage. I feel like a total moron. How could we have been playing this wrong all this time?
((Not that I'm complaining, having been the spellcaster in question, but still!))