remember one match where it was a beautiful day but had rained the night before. After 20 overs of batting,one of the oppo hit the ball for 4 and it rolled into the covers on the boundary. The ball rested in a puddle of water. It was dried off but for the next 2 overs it swung round corners, the poor lad bowled outswingers normally but for the next few balls it ducked in and down leg big time.
Going back 20 years ago we brought some cheap Indian balls for Sunday matches. On one occasion our home ground & pitch was absolutely bone dry and rock hard. Not surprisingly the cheap ball was knackered after 15 overs. This was the time when both teams used the same ball, so you can imagine the oppo who were bowling second weren't impressed. They got 240 odd batting first and we were 80-2, when all of a sudden this lad starting swinging it round corners. It all ducked in and went down leg but the bowler kept saying "I bowl outswingers normally, I don't understand it". He got taken off not long after. I took the match ball and played around with it in the nets and even at my modest pace it reversed, probably due to the totally shagged nature of the leather than any skill on my part. It's the only time I've seen a ball do that in 30 years.