When I had to postpone the release of v2 of devRant UWP for 3 months because I changed a few minutes before the final build a "loaded" event to a "loading" event.
Something that for some stupid reason I thought will make the startup faster (maybe by 2ms) has caused crashes on mobile and some Windows 10 versions.

Of course I didn't spend 3 months to find this bug, but at least a few days in December and then I didn't have enough free time so I had to stop until mid February when I finally fixed it.

To find the reason I reverted the code to the previous update and started to add every feature one by one.
So much time wasted for a fucking "-ing".