Me and my mum were watching Mamma Mia 2 (kind of spoilers ahead!) in the cinema and we were at then the bit at the end where Sophie's baby is getting christened and Donna's ghost sings with Sophie. Very emotional and sad. Randomly my mum whispered to me "doesn't Glenn Close look good for her age?". I was
and whispered back "I'm not sure I've not seen her in a while". Then mum looked at me like I was crackers, pointed to the screen and said "She's there.", meaning Meryl Streep. I burst out laughing, which made mum laugh and boy did we get some funny looks from other (teary) cinema goers probably wondering why the hell we were laughing at such an emotional scene (before I get flamed I did apologise and made an effort to quieten down). I felt really bad but I really couldn't help giggling.
Also when I started my first office job she 21, my first week ad I was really nervous. it was really stuffy in the room and my supervisor went to open the window. She opened it onto her forehead, and I started to laugh before I realised she'd actually cut her forehead as there was blood gushing down her face 
Please regale next with your cringey inappropriate laughing moments!