I always marveled at the details put into trolls.
For example, modern trolls just tell us this about regeneration:
"Regeneration. The troll regains 10 hit points at the start of its turn. If the troll takes acid or fire damage, this trait doesn't function at the start of the troll's next turn. The troll dies only if it starts its turn with 0 hit points and doesn't regenerate."
But original trolls give us all of this:
"Trolls regenerate at an amazing rate. Starting three rounds after first blood, the creatures recovers 3 hit points per round until healed. Trolls reduced to 0 or fewer hit points fall to the ground, incapacitated but not slain. Incapacitated trolls continue to regenerate and stand up to fight as soon as they have a positive number of hit points.
When using an edged weapon, it is possible to sever the thin limbs of a troll (a natural 20 with an edged weapon is needed). Severed limbs continue to fight after separation from the body (hands squeeze, heads bite if stepped on, etc.). Attacks by severed limbs are at normal chances to hit. Separated limbs fight for the remainder of the battle, then scuttle back and rebind with the body once the battle is over. Limbs unable to reach the body to die within 24 hours, but this is of little consequence since trolls regenerate lost body parts (including the head) within a week. If a troll is dismembered and scattered, the largest surviving piece regenerates. The others die within one day if they cannot rejoin that piece.
Only fire and acid cause permanent damage to trolls. These forms of attack destroy its regenerative ability. A troll reduced to 0 or fewer hit points and immersed in acid or burned with fire is killed."
If course, this section is all under the label "Combat", so it's harder to find. And more of a PITA to adjudicate. And there's also stuff about getting three potions worth of blood from a troll with no explanation of how troll blood regenerates. And it's not necessarily consistent with other crit rules. And it's hard to remember all the details. But it's there when you need it.