When a fluid tank is used as a buffer, as in the image below, it will keep sending fluid into the output pipe, even if there is nowhere for that fluid to go.

I would expect the tank to start filling up once the electrolytic separator can't accept any more water. Instead, any water pumped into the tank at that point is simply lost.

Complete list of mods on my testing system:
Minecraft 1.10.2-forge1.10.2-12.18.1.2092
Mekanism-1.10.2-9.2.0.292
MekanismGenerators-1.10.2-9.2.0.292
MekanismTools-1.10.2-9.2.0.292
MCMultiPart-1.2.1-universal

Just did some testing, didn't run into any problems. Note there's a buffer in Mechanical Pipes.