Custom furnace is made especialy to exploit fluid engine limitations. From top there is regular pipe connector in group with underground connector from upper right. From bottom there are two isolated underground connectors. My goal was to create setup where there are no blocked undergrounds (so teleport is not rejected because of entity being connected to blocked fs), but i wanted furnace to be teleported out of this setup where it would mix fluid and so teleport should be reverted but fluid systems would be already mixed so even rejected teleport would perform mixing. Connectors are specialy ordered so that removing entity itself should be enough to do fluid mixing. Intended scenario: bottom two furnace connectors disconnecting first, allowing outer bottom pipe-to-grounds to connect (blocked), then removing upper right furnace connector doing block exchange(as in sneaked gif in 74614) from bottom underground pair to upper underground pair and then rejected teleport would connect upper pipe with upper right underground doing mixing(exactly like in 74116). Something went wrong and instead game crashed.
*cough* "deferred block fix"(viewtopic.php?p=450912#p450912) *cough*