[jacquesm]

Yes, those are problematic. This tends to happen when a pipeline changes payloads. I know a little bit about this: typically what they try to do is to shuttle the 'interface' to a segment long enough to contain it completely, then vent that section by opening a valve at the end of the section, then push new material in until the mixture reaches a certain level of purity.

Unfortunately, the mixture is useless and vented into the atmosphere. There ought to be a burden of responsibility to deal with that in a more ecological way on the pipeline operators, but these tend to be in bed with the nation states that are home to the pipes.

A pipeline carrying a single substance or single mixture of substances can still vent, usually that is to deal with pressure excursions and is a safety measure.