For several weeks now, unscrupulous Overwatch players have been able to take advantage of a glitch in Mei's Ice Wall that puts her into normally-inaccessible areas of the game, from where she can attack but not be damaged. Naturally, that's a real game-breaker, yet Blizzard has had real trouble tracking down the problem. There is a fix in the works, however, and it may even be rolled out today—and in the meantime, game director Jeff Kaplan warned that players who take advantage of the exploit will be punished.
"This bug was surprisingly tricky for us to track down but we believe we have identified a fix. Our hope is to have it fixed sometime tomorrow (1/5/2017 PST) but there is always the chance that our fix could fail. We're currently testing to verify the fix," Kaplan wrote.
And then, the warning: "This bug is clearly an exploit and we consider this cheating. Do not attempt to reproduce this bug. If you see someone abusing this mechanic, please report them," he continued. "We will be taking action against those who abused this exploit. Overwatch is a PvP experience so that means exploiting game mechanics like this comes at a cost to those you are competing against. We take aggressive action against people who abuse game mechanics, hack or cheat. This case is no different."
At last check, the fix hadn't been deployed, so the exploit presumably still works. I would strongly suggest staying away.