Here are a couple of rules from last two games.
Tally of Pestilence, 6th ed Chaos Daemons Codex pg 52
Whilst Epidemius is alive, keep a count of all unsaved Wounds caused by the psychic powers, shooting and close combat attacks of Daemons (friend or foe) - even those that are negated by means other than saving throws, such as Feel No Pain.
Context: This was supposed to be about how Soul Grinder's lose of Hull points count towards Tally of Pestilence. However after reading this my opponent and I ran it wrong. Guess we were using 3rd ed codex. He was counting number of Deamons of Nurgles took wounds not caused.
Consolidation, 40k6 pg 27
At the end of a combat, if a unit's opponents are all either destroyed or falling back or the end of combat Pile in was insufficient, so that the units are no longer locked in combat with each other, they may Consolidate.
Context: One squad of Purifiers wasn't in range to Pile in with Scarabs. They consolidated. Connor started consolidating them and I was a bit confused. He reminded me of this rule.