Can it be sent accidentally?
No it can't. 7500 has to be dialed on the IFF transponder and the "squawk" button has to be pressed.
I can't imagine how, but yes, I'm sure it's POSSIBLE.
Someone upthread did point out that anyone managing to do this accidentally is going to be in a world of sh!t once on the ground.
Yes it can. The code is 7500. If you're sqauking 6500 and told to go to 7400, you should first change the 5 to a 4, and then the 6 to a 7, so you pass through 7400 for a second. If you change the digits in the other order, you pass through 7500 before getting to 7400 ... this apparently has happened. I'm not saying it's the case here.
I have never heard of it being squawked accidentally. The only codes starting with 7 are "emergency," "hijack" and "lost comm," I think. I don't know if they have it on speed dial nowadays.