I used to share flights with the owner of a C172 who had an IMC rating. This meant that I would sometimes fly in conditions which I would not have risked on my own, believing that he could take over if necessary.
One day, I was flying the leg out to LFAT and was confronted with a thunderstorm beyond which I could see brightness that suggested it was quite a small one. We entered very heavy rain, the air was quite smooth but ahead of me I could see a large inverted dome of cloud. It looked very smooth, almost solid. I decided to give it a wide berth, probably the only sensible decision I made that morning.
We eventually emerged the other side into bright sunshine and a pleasant flight to our destination.
If I hadn't gone flying that weekend I would have been sailing where that storm was centred. I asked a friend who did actually go what the weather was like on the ground. Apparently, Cowes High street was under a foot of water.
So this time, rain was the main hazard but I hate to think what would have happened if I had got close to that cloud. I learned enough to ensure that I would not do it again.