The year is 2004, and the 4th edition of the Warhammer 40,000 Rulebook is released, alongside the Battle for Macragge (which had a whittled down version of the rulebook).
One of my fondest memories of this edition was the "target priority" rule.
It was an attempt to try to capture the sheer immediacy of battle. If a unit wanted to shoot at an enemy other than the closest one, it had to take a leadership check to be able to do so. I personally really enjoyed this rule, even if it did make the game slightly clunkier, as I felt it really captured what a tabletop war-game should be in essence.
The downside was that most units would have at least a leadership value of 9, meaning that these kinds of rolls were marginally pointless most of the time. Equally, in several tournaments, forcing my opponents to use this rule really saved my bacon a few times. So it is one of those rules that has really stayed with me. I genuinely liked it, and wouldn't mind seeing some kind of analogue to it reintroduced (if some way could be devised to do it so that it wasn't just another pointless dice roll!).