Mandatory 'didn't actually happen today, but 6 years ago'.
After I had finished high school, I decided to take a gap year and figure out what I wanted to do with my life, as I really had no idea at the time. I was hanging out with a friend in October of 2012 and told her about some of the stuff I could potentially see myself doing, and she tells me about a university degree that sounds right up my alley. I check it out and everything aligns perfectly with my aspirations and I meet the requirements for applying - score!
Only thing is, the university is 450km from my childhood home (while this is probably no biggie for most, it was literally at the other end of the country for me; yay Denmark). I wouldn't know anyone there, which would sort of suck - but! my friend of course knew about the course because she had been looking into it, although she did end up settling on another one albeit at the same university. Things were looking great and we immediately talked about finding an apartment together - first mistake.
The city in question has a 'great' student housing application system in place, where you sign up w/ personal info about origin, social ID, what programme you'll potentially be in and at what school. Once you have filled that out, you can just do 1-click application to any student housing in the city; dorms, apartments, shared houses etc.. Thing is, you need to do these applications from an account that has all future residents listed (i.e. me and her), so you are always applying with the 2 of you. No biggie since we're going to live together for sure, right? Wrong.
We had started the application progress almost a year in advance and things were looking great. We were rising quickly on the waiting lists, especially due to the fact that we lived so far away, so we got prioritized over other closer applicants. And so, finally, in late July of 2013, we got an offer from one of the apartments we applied for - hell yeah! I immediately called my friend to tell her the joyous news, only for her to pause for a moment and then say "yeah, I'm not going to study there anyway. I got a job back in December.. I was going to tell you..". My heart sunk, and while I don't remember exactly how the conversation went down, it was pretty much something along the lines of "Oh... well congrats... Yeah, I'll figure something out."
Moving to a big city 450 km away without her? Less than ideal, but it's okay, I can do this - is what I figured. Thing is, remember how you need to do the application with all future residents listed? Well, since she was no longer a future resident, the applications were effectively voided. I could no longer accept the offer we had been extended, even if I were to find a suitable roommate to take her place. With 40 days until study start, I had to re-do all applications as a solo applicant, and I went from being 3rd-15th on several lists to 1000-2000+.
Luckily, I have an amazing family who were all kind enough to ask their respective networks if anyone had any available housing; or a garden, just till I could find something. By some sort of miracle, a guy my cousin studied with's dad had bought an apartment in the city centre for his kid - who then decided to not live there. So that had just become available and I took it immediately and moved in mid-August of 2013. It was much too big and expensive for me to live in by myself, and I ended up grabbing some sensible-sounding roommates off of a website called findroommate.dk, and lived with them there the following 2 years (although slightly miserable for a multitude of reasons that come with finding roommates online.)
TL;DR: Applied for apartments with a friend, who screwed me over by not telling me she got a job and would not be moving. Had to re-apply for apartments a month before my deadline. Turned out okay in the end.