Black no entry sign

Simple question:
Sometimes when I am mapping variables in the pipeline, the cursor turns into a blank no entry sign, i.e. black circle with a line through it. I assume this means something is wrong but what should I be looking for?

I’ve experienced in Developer when stepping through in debug that the cursor gets stuck in the wait (hourglass) state. When it does this, the ability to click on anything goes away. I wonder if these two symptoms have the same root cause?

i have seen this happening to me quite a few times ,i usually disconnect and log back. but if someone can explain why this happens ,it would be much appreciated .