Regen does not regen extra hearts added by Health Boost. Also, if regen level is over 2 after you give yourself level 255 regen, you will take damage.
Steps to reproduce 1st bug:
1)Give yourself health boost (/effect @p 21)
2)Give yourself regen (/effect @p 10)
3)Note that the regen is not regening the extra hearts.
Steps to reproduce 2nd bug:
1)Give yourself regen of 5 (/effect @p 10 30 5)
2)Note that you are not taking damage.
3)Give yourself regen of 255(/effect @p 10 30 255
4)Note that you are now taking damage, even in creative mode.
5)Give yourself regen of 5 (/effect @p 10 30 5)
6)Note that now you are getting damage even though you have the same effect as step one.
Environment
Mac 10.6.8, java version "1.6.0_43"
Java(TM) SE Runtime Environment (build 1.6.0_43-b01-447-10M4203)
Java HotSpot(TM) 64-Bit Server VM (build 20.14-b01-447, mixed mode)
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