I recently reinstalled The Binding of Isaac and attempted something special:
Having played through the game once already, I decided I would give myself a greater challenge by attempting to beat the game (get to the chest and kill ???) while bypassing Mom every single time. This means that I should never get the main ending and unlock chapter 4 itself.
And sure enough, through the help of 9 lives, I quickly did beat the entire game like this. It had some interesting effects on how I unlocked things.
First of all, it is possible to beat mom but not unlock the secrets concerning the womb. Just teleport out of the room or simply exit the game after beating her, before the actual ending starts. It will however get you stuck on the level, and yes, it still drops the polaroid, without spawning a trapdoor however. This will be important concerning the challenges.
Now, I was unable to select Blue Baby as a character as if I hadn't unlocked him, but I could still clearly see him. The ending in which you unlock the character also for some reason didn't show up, but I could still play as him by glitching the select screen. On the other hand, all challenges which would normally be locked when not having gained access to the womb were unlocked after I beat Isaac for the first time. This is probably connected to the final challenge unlocking by beating the cathedral, having the nifty side effect of unlocking all the others too. It was also possible to select them, so it wasn't just a graphical glitch, but the level intermission screen still showed only the depths 2 as the final level. Now comes the interesting part:
I played the "Meat Forever" challenge, with which you'd normally have to beat Mom's heart to win the achievement, and of course I got the Cube of meat before it was unlocked. I was hoping to get either some telepills or the shovel to bypass Mom, but neither did happen. So when I eventually got to the Depths 2, and decided to kill mom anyways, teleporting out of the room as usual. Normally, you'd get stuck there at that point, with no way of continuing, forcing me to restart
But to my surprise, the secret for the "SMB Super Fan" item unlocked right after I beat mom, instead of beating Mom's heart. This is quite a useful exploit if you're crazy enough to beat Mom's heart 11 times and kill Isaac while bypassing Mom herself. I'll have to check if it works, but it could also be extremely helpful for getting the Purist challenge done without having to beat the last 3 floors.
Probably not very helpful for anyone here though, but it's still interesting to see how much the game can be broken