According to the wiki, lava deals damage every 1/2 second, which is verify-able in game by jumping in some lava, not an issue. However, when the player has regeneration on him/her from the /effect command, it seems to deal damage at the rate of the regen rather than what it should be.
Steps to reproduce:
type /effect your-player-name 10 (for regen) 15(any duration long enough to stand in lava for a few seconds) (a level of regen, minus 1)
example: /effect name 10 15 0 : applies regen 1 for 15 seconds to player "name"
Stand in lava.
Observe the speed at which you take damage.
When refering to regen here, I mean the displayed value in the inventory, not the value you put into the command.
Regen 1 causes very slight damage speed increase, nothing many players will notice.
Regen 2 is much more noticeable, and the highest a player can normally get "legit".
Any higher, and the regen damage bug gets much more noticeable, and I believe around regen 10 it is near instant death from full health.
I have not tested this with regular regen potions, although since it is just applying a potion effect, one would assume the issue is the same.