Have you ever made an illegal move?

Yes I did. Once in my school chess competition, it happened but not to me. There were 5 games. In the 3rd round I was leading with all wins. Then my opponent made an illegal. I did not see it and continued. But then when I realised, I analysed it with my opponent (at the same time) and then told her about it. She did not know many rules (nor did I , cause if I did, I wouldn’t have done it) and accepted it and I later went on to go win that game by 2 illegals.

That tournament I was second and I won 4 games out of 5 on illegals!!

Thank you for seeing my story

Regards,
pdawn

Yes, I think.....

I played chess with a long time control in a tournament with a weaker opponent. I was tired but thought I would win easily. In the opening, I castled to the long side and my opponent to the short side. I made a combination at the end of which I had to checkmate on h7 with a queen. I could not believe that my opponent does not see such simple things ...Bg5+ Qh7!#. When my opponent stopped the clock and called the judge, I didn't immediately understand what had happened, but later I saw that the bishop from g5 declares check to the king on c1. For the illegal move, they did not bet a defeat and in the end I won the game.

Yeah, in my first tournament I lost a game by illegals moves because I didn't see the checks :( I was winning the game...

noooooooooooooooooooooooooooooooonb

The rules of OTB blitz state that an illegal move by a king forfeits the game...of course that is not possible online

In my college days a variant that was popular was OTB, 5+0, and allowed to capture the King if the opponent leaves it in check! If the King is captured the game ends. So technically leaving your King in check is not an illegal move in that variant. Your opponent might actually miss the King capture. Since one of the ideas of the variant is to get better at board vision, you really wanted to see that your King was in check. Many games ended with King captures until the person got the necessary vision. As a novice to speed chess at that time, and vision not what it needed to be, I'd miss discovered checks and get my King taken.

"Have you ever witnessed people making illegal moves?"
Yes. Novices make illegal moves OTB often. Not even talking speed chess. They leave their King in check, castle incorrectly, etc. In our club it is not unusual for novices playing each other to ask a more experienced player to watch and make sure they play a legal game.