I know of three visible defects in this picture. One is obvious; two are less obvious but more important.
I’ll give you one of the less obvious ones because I didn’t intend to include it in the photo and not all of the info is readily visible: Notice that there are both Square D and Cutler Hammer breakers in this Square D panel (the CH breakers don’t fit properly).
That leaves the obvious and not so obvious defects involving the center breaker. Everything you need to see to identify the issue is readily visible in the photo.
I’m posting this for discussion as it is a problem that I see on occasion (usually when I’m doing an IR inspection of a panel), but almost never hear discussed and it does present a potential fire hazard.
What is the issue and why is it a problem? Experts may wish to wait a bit before dropping the answer.