I don't really know if this is a bug, but I was messing around with the "HealthBoost" effect and found out that if you splash yourself with a Amplifier 252 Potion (wich is equal to -4 since it's a byte) you lose Health, but if you do the same thing with the command (/effect @p 21 60 252) you get more Health...

I think it would be nice if it was also possible to give you a negative effects with the command, since it's a bit anoying, always having to summon a potion using the /execute command