Since I see many people making that mistake, I have decided to make a specific thread relating to it.
So when you click Hide Details it creates this nice line that you can click on to uncover hidden stuff like this:
Summary
#Hi
But now notice how it breaks so the the stuff you want to hide is uncovered like this:[details=Summary]#Hey, why I’m uncovered? D:[/details]
To fix this, you need to separate it each time you add it by an additional line by hitting Enter like this:
[Hidden Details]
[Hidden Details]
Here is the version that you shouldn’t do:
[Hidden Details]
[Hidden Details]
I hope this little tutorial improved your life at least by a tiny bit 