Disclaimer: Don't own and never will.
Even by Chat Noir standards, this was spectacularly bad luck.
It wasn't slipping on the rooftops that was the problem. No, Chat easily caught himself with his staff thanks to his cat like reflexes. (Which could have been a cat-astrophe otherwise, he thought to himself.)
It wasn't even the window he unfortunately broke as he went through an unfortunately closed window.
No, what was spectacularly unfortunate was the fact that he landed in the middle of a slumber party at Marinette's house just as his transformation dropped. That was a surprise.
And judging by the looks on Marinette, Alya, Rose and Juleka's faces, he wasn't the only one.