Code Ganking

I have yet to ever figure this one out. I implement a bit of code on a website. It works. I then yoink that code to use in a similar fashion on another website.

The funny thing is when everything gets put in place it doesn't work the same. Nothing has changed except for a few dimensions and maybe some CSS code. I stare at it for hours changing every little thing and yet NOTHING. Starting from scratch does not appeal to me at all. WHY IS THIS FUCKER SITTING 10 PIXELS LOWER THAN THIS OTHER ONE!! IT'S THE SAME DAMN CODE!!

So after you get done weeping into your keyboard, you take a deep breath and spend more hours scouring the internet for an answer only to come up empty handed. So you make a test page with JUST that piece of code and it works fine there but not in your current layout. Why? Who the fuck knows!?

Finally you just cough up the 8$ for the most current version of the code and realize from the examples that the exact same CSS applied to a class instead of an ID is the only way to get it to work. Why? I have no idea. All I know is that it finally does what IO want and I am TERRIFIED to look at any other browser at this point for fear of an aneurism.

Hypothetically of course...