I was building a small brick building for a parkour challenge and it started raining. I made a ceiling for the place to block the rain. Now, normally it would show little drops going slowly (drip drip). But it started raining inside the place. It was taking a couple seconds the spawn the rain, but it was just one drop, so I could not take a screenshot because the drops were fast. I think the game doesn't properly load the drip drips in a small space. So it actually loads the drips as rain.

How to replicate:
Make a small building with a flat roof.
Wait for it to rain.
Look at the line where the left wall meets the ceiling. Sometimes you will see a quick drop of rain.