"Resolved before it was meant to be"

The error reads roughly:

Cannot mutate the dependencies of configuration ':<module>:<someVariant>'
after the configuration's child configuration ':<module>:runtimeClasspath'
was resolved. After a configuration has been observed, it should not be modified.

You get it from a line as ordinary as dependencies { someVariant("group:artifact:1.2.3") }. The build worked yesterday; the only change is that one line, or one newly applied plugin.

What is actually happening