It depends on the hostel of course and I often just opted for a hotel room, I rarely had anything of note happen when traveling alone but with my friends other stories, but the only ones that stick out in my head at all are:

Berlin - Just particularly nice and clean with lots of chatty English speakers, first tongue or very proficient.

Munich - Was with friends from the service, and lodged with other younger guys from the service we didn't know, came in by myself later on to a hostile quiet silence they explained later, apparently things nearly turned bloody in my absence Noteworthy because I apparently fell asleep clutching a nearly full bottle of beer and impressed my peers that I didn't spill it and drained it within minutes of waking up.

Rome - Very crapshack, but co-ed room where a couple of young ladies in ours changed in the morning with zero modesty and a touch exhibitionism... which I also missed, having already awoken and downstairs sipping coffee at the time but my friends drooled about it all day.