In some cases liquids do not act like liquids. See the image below and the strange spiky shape of the liquids.
I think I understand the technical reasons for why it acts this way. Each layer is a different liquid (water, polluted water, petroleum, crude oil) and they don't mix, so each stays on its level. And since despite the look of it there's only very little of the 3 top liquids, they do not spread to the sides. But still, this is rather silly and also annoying, as it blocks gas flow between the left and right side. I think this could be handled e.g. by liquids trying to fall also sideways, or if a liquid tile has another liquid on top of it and there's room to the side, this lower layer could move to a side.
Steps to Reproduce
Stack small amounts of different liquids on top of each other.
This issue has not been confirmed by a developer yet.