As a frequent business flyer ( 126 flights in the last twelve months alone )who is scared ****less of flying most of the time I've only ever seen this happen once on an aer lingus flight from Dublin to Manchester about 10 years ago.
The plane landed and we were heading for the gate, what at first appeared to be smoke started appearing at the side of the ceiling ( sorry i'm sure there is a technical term )
At first I thought it was smoke but remember that smoke accumulates and steam dissapates so I reassured myself today was not my day.
But someone else shouted "jesus we're on fire...." and panic ensued, after a short while the pilot came on the wire to explain it was steam due to high humidity
Now in all my time flying as a pax i have never ever heard a pilot pro-acively explain this item.
I've only seen it 3 times,