tl;dr be nice to your knees.
The first time I went to Europe, the very first day we arrived, we'd been exploring an abandoned airport for about 7 hours and had just gotten to the back portion where the hangars were. The grass was a little annoying so we thought it'd be easier to step down into the channel next to the tram line and walk that way - but it was dark and I misjudged the depth, so ended up jarring my knee wearing a 12kg pack, and fell on my face with my DSLR in hand. It felt like I'd broken my knee, I'd heard something pop and felt something twist inside it. After a while I managed to stand and limp out of the airfield, and we continued our 2 month eurotrip with me hobbling along the whole way. I later found out this was ACL tear #1 - #2 and #3 were equally uneventful, both happening when jumping over a fence, and numbers 4 onward will come from something else boring I'm sure. The human knee is terribly designed and fragile, so make sure you take good care of yours!