Hmm I believe I have 'the' most perfect example of that in a recent game. The Binding of Isaac: Rebirth.
Anyone unlocked 'The Lost' yet? Did it accidently? Nope, didnt think so. I wont even tag this with a spoiler warning seeing as the chances of unlocking it 'accidently' without checking up online..are roughly the chances of finding a snowball in hell, being revived with a winning lottery ticket in your hands and being crowned emperor of the universe shortly after...all in the same day. Ah-hem, if I may...
Step 1: Die to a Mulliboom with Isaac on the first 2 floors.
Step 2: Die from your own bomb with Magdalene in floors 3-4.
Step 3: Die from Mom (only the boss, mobs that do damage in her boss fight do not count) with Judas.
Step 4: Die from Satan with Azazel. The Fallen beforehand does not count.
Ok so those are the steps, but wait..theres more.
-Upon each death you must exit the current run and click on that other character.
-Its been patched so seeds are not allowed on ANY characters ( atleast as far as Ive come to understand..seeing as Ive only been using the seed for Isaac to get one to spawn..upon completing it with Azazel twice now completely...Im assuming this is what caused it,good luck getting a Mulliboom to spawn...)
-If you exit the game completely, you have to start all over again, so this all must be done in one conesecutive run.
-If you die from anything else at any point you have to start the process over again from Step 1.
Now you may ask yourself 'what the hell? how was I even remotely supposed to find that out?' Well my friend, Ive got news for you.
'Dying in a Sacrifice Room while holding the Missing Poster would yield a puzzle piece on Isaac's last will. These puzzle pieces, when put together, created the last wills of characters dying in the method used to unlock The Lost. '
Yep, thats all I have to say for anything consisting of 'How the hell..was I supposed to find that out?'.
Edit: Ill just add, for anyone wishing to grab it nowadays, if you press escape during the death animation (if you died to anything else) and hold down the r key, the current run will reset and you wont lose the 'progress', aka, if you die to...I donno, It Lives with Azazael on accident, you wont have to start all over again with Isaac.