I’d imagine that most Dungeon Masters have made their own tweaks and variants to the Dungeons & Dragons game system. The most popular of these “house” rules often spread to games run by other DMs. Unfortunately, popular doesn’t always mean good.
One example of such a flawed rule is double-rolling hit dice, wherein players roll twice for each hit die and take the highest result. I got this one from my former Dungeon Master, and used to use it in my early games. It’s clearly popular among powergamers, and at first sight there’s nothing wrong with the rule.
Mathematics disagrees! Statistically, double-rolling increases the average result of hit dice, making characters tougher. A fighter’s d10 rises from an average 5.5 to 7.15, an increase of 30% before Constitution, and with 16 Constitution still around 20%. The problem with this, as opposed to something like higher ability scores, is that you throw hit points out of whack with everything else in the system. Your saves and offensive abilities are not improved, only your capacity for taking damage.
In this case, it’s especially bad because more hit points has the only effect of letting you last longer and relax a little in combat, taking an element of risk out of the game. This has the effect of making combats last longer and making the game more dull, the latter of which is a Bad Thing. If you scale up the opponents’ hit points by double rolling to compensate, it just exacerbates the effect and makes extra work for yourself, and if you scale up the opponents’ challenge ratings you’ve simply made the game more deadly because your players’ offensive and saves don’t match their hit points.
Beefing up your player characters for its own sake isn’t necessarily good. If it just makes the game easier for your players, it’s not necessarily going to make the game more enjoyable.