For me it was definitely the time in 8th grade when my parents had their first and only physical fight. To explain my parent's relationship, my mom is a cunt who verbally abuses my dad daily (on the days she's actually willing to speak to him) and pulls tons of crazy shit that he has tried to stop but at this point just sit backs and takes. She's not exactly a peach to my sister and I either but sometimes she puts in effort to try and get us to like her. My dad has anger management issues, so every few months he blows his top and they then have a shouting match followed by my mom giving him the silent treatment (so mature I know).
I'm unsure of their weight but both of my parents are big, with my dad at 6'4 and my mom shorter at 5'7 but quite a bit overweight. I had been hearing a screaming match from my upstairs bedroom, which wasn't out of the norm but then it got loud and I heard banging around, which I had never heard before so I sprinted downstairs to the sight of my dad covered in scratches looking like he was planning to choke the life out my mom. I'll never forget that look of rage that had been in his eyes... I started screaming at them to stop, and started crying as I got in between them and forcefully pulled at my dad's arms to get them off of my mom's neck. Naturally my mom took that chance to throw herself at him again, so I had to push her away (that had been easier for me because I had 2 inches on her). This ended the fight, but made the rift between my parents turn into a gaping abyss where their previous need for a divorce became desperate, but still has not occurred after 5 years due to financial constraints.
That night of their physical fight was a night both of them had been drinking heavily. After that day my parents rarely drink alcohol but whenever my dad does decide to have a few beers my mom always makes a point of trying to enrage him, though thankfully after years of practice my dad can mostly ignore her