You want to measure a distance in an image. You select the line tool. You set the first point but then you figure out that you want to measure in the next layer because on this layer the landmark is not clear... So you scroll to the wanted layer. But now you cannot draw a line because it is greyed out.
So what happened is that the measurement tool is still active on the previous layer. That is also why the user cannot create a new measurement.
Either blocking the changing to another layer or removing unfinished measurements when changing.