I wouldn't say that I learned something new, but more that it reminded me that you should never believe everything will work fine even if you made it dozens of time in the past and it always worked. I tried to implement a saving/loading high scores system in my game which is literally just saving 3 Lists of 10 integers and because I tried too hard on making the high score "secure" in a certain why I'm normally used to use, I couldn't make it load it back in even though the save was successful. there was some error code popping out telling me that the list was not created (while it truly was as I could access it internally) and due to lack of sleep and having wasted 5 hours on debugging the crap out of that broken system, I scrapped the score board for the Jam.
To give an idea of how much I'm normally used at managing this kind of system, I have made a save/load system for an action RPG that save the data of 8,999 NPCs and 1 player and the whole system is quite solid in a way that makes data injection pretty much useless. (The save file can be edited, but even that comes with back-checks when the files are loaded and even saved.) And yet, in this case, I failed at being able to save and load (and use the data in-game) of 10 scores.