I fondly remember the one and only time I witnessed an honest-to-god actual rout of PCs.
It was decades ago, so I don't recall all the specific details, but the game was Rolemaster, and the party had encountered some kind of incorporeal undead, which it became clear they could not defeat. The only open avenue of escape was up some stairs. As everyone was moving at pace, I had them make manoeuvre rolls to confirm what actual speed they were moving at. Then, when faster people were stuck behind slower ones, I asked whether they were pushing past ... as soon as one was, they all were. Someone fell down. Did anyone stop to help? Nope, we're just carrying on over the top of them, because if we stop we die. I don't know if more than one PC ended up dying, but I definitely remember the chaos and the every man, woman, dwarf and elf for themselves attitude that just emerged naturally.
I don't think I've ever seen anything else quite like it.
I have had a small number of heroic last stands, where one or more characters give their lives so the rest of the group can escape. One was a lone character holding back a dragon, also Rolemaster, back in my high school days.
More recently, playing Hackmaster, the PCs bit off more than they could chew, and by the time they realised how dire the situation was, it was far too late for any kind of orderly withdrawal. However, one or two PCs and a henchman did what needed to be done and, with a bit of luck, careful positioning and valiant selflessness, they were able to enable the rest of the group to escape, laying down their lives in the process.
More recently again, and back to Rolemaster, the PCs took on a demon that greatly outclassed them. They had managed to reduce the demon's minions down to the point they were manageable (and where victory seemed within the realms of possibility). When the enemy reinforcements arrived, the PCs were just barely able to disengage and escape before they were overrun.
I do agree, though, that usually, orderly retreats are next to impossible unless they're done immediately.