Main Ckt Brkr Tripped

I inspected an Siemens (1991) 150 amp electrical panel this morning and as I put the cover back in place the Main Ckt Brkr tripped. Scared the crap out of me but no shock. My first thought was I must have done something wrong but after awhile I came to the conclusion that I did just as I always have and maybe there is a problem with the panel itself.

Any ideas?

Did you flip it back on?

erm… I flipped it back on after I put the cover back on.

Most likely you accidentally tripped it when putting cover back on, it doesn’t take much.

Any pictures of panel with cover off?

I added a picture to the first post.

I have done this a few times also .

The best one was a few years ago Just as I put on the Cover No power .
Dam I thought I had tripped the breaker .
Removed Cover and tested no power .

OH! OH! Looks like I tripped the Line coming in from out side Now I am in a bad mood.
Just then My cell Phone rang , My best friend said have you heard all the power is out in Southern Ontario and northern NY.
Well this sure got my attention now I can see Headlines Home Inspector has Got him self in how water.
Then I realized NO Way could this be me and then I saw the humor .
It was a resistor in Niagara Falls that had gone bad .