These have a lot more piping than oil tankers, so each cargo compartment can be loaded with a different industrial commodity, some of which are known to be highly incompatible, such as acids vs alkalis. There's usually 10 or 20 different chemicals on a vessel at any one time.

This way if they are careful, the operators on board and on shore can transfer the parcel, without significant enough contamination from foreign chemicals in other tanks, otherwise the material would fail to meet specifications.

Not everyone is as careful as they should be all the time.

When a cargo does go off-test it can take a lot of prime material to blend it with in order to pass.

It can be a headache of industrial proportions. Lloyds of London can get involved.

For that reason I hate it when a benzene or methanol transfer picks up a little too much palm oil.