Damage taken can reset/slow velocity in some cases.
I did a quick test where I swam up lava, and with fire resistance, it took about 4 seconds. Without fire resistance, where I took damage, it took about 5.5 seconds to reach the top.
It is also slightly noticable when jumping with wither/poison/fire, your jump can inconsistently be shorter than normal.
MC-3813
Damage taken resets/slows player velocity under some circumstances
Resolved
Works As Intended
0
(Unassigned)
Unconfirmed
damage, fire, lava, poison, swimming, velocity, wither
Minecraft 1.4.5
Retrieved
This is intended. It wouldn't make sense for the player not to be affected by damage.