Kerbal Space Program

there are times when it is preferable to draw fuel from the same tank in two different ways

this craft suffers from the problem that one of the tanks attached to the fuel tank under the mk2 cockpit will use up its own fuel before drawing fuel from the big fuel tank. This creates an asymmetry in the craft and can lead to unstable designs and incorrect fuel distribution when staging.
Testing has shown that if an active engine draws fuel from the part where the mk2 cockpit is the fuel flow will break and one of the tanks will drain before it is intended, if no active engine exist on the cockpit section fuel flow as expected. If fuel routing where done independent for each engine this problem would no longer be an issue. Since the path used is irrelevant as long as the fuel drains in the right order. The way the system works now is not intuitive and cause a lot of confusion and sorrow.

this issue is referred to in the following forum tread as rule 1. http://forum.kerbalspaceprogram.com/index.php?/topic/58458-fuel-flow-rules-0242/&page=1