BuildCraft version: 7.99.17
Singleplayer or multiplayer: both

Consider the path

oil (cold) -> Distiller -> Heavy Oil (375 mB) + Fuel Gas (2 B)

If the 375mB of Heavy Oil is sent to the Heat Exchanger (HE), the HE it will only consume 370 mB of the fluid. More importantly, the remaining 5mB will be stuck at the entrance of the HE, causing no other kind of liquid to enter until e.g. 365mB of the same liquid arrives to the HE.

IMO, the HE should probably not store liquids on its ends and instead only accept liquids from the input pipe to the output pipe (with cooling/heating). This facilitates using the HE on more complex and fun designs where it is being used to heat different types of liquids during operations.