Anything in from stuck pins which prevent the computer from charging to my cats deciding the adapters are made out of some rare form of addictive kitty-crack and shorting out the cord by chewing on it.
I used to buy new MagSafe adapters but that got expensive fairly quickly. Then I decided to splice out the shorted sections of cord; which did work but left me with relatively short cords for charging the computer. Next, I decided to figure out how to crack open the adapter and replace the entire cord and that's the process I'll demonstrate with this instructable.
Be careful and make sure the adapter is completely discharged. Capacitors can hold a charge for a while so it's best to leave the adapter unplugged for a bit prior to working on it.
Needle Nose Pliers
Clamp(s) or even a heavy book will do.
Replacement MagSafe cord- I found mine on eBay for like $16.
Step 1: Opening the adapter
Flip open the cord winding ears and insert the plier ends as the picture shows.
Open the pliers until you hear a satisfying pop and then do the same on the other winding ear.
This will typically separate the two halves almost completely but the case will still be held together near the wall plug and along that back seam. To get those to release you'll need to grab the two halves and pull them apart until those also pop apart.