I am fairly well known for always having my laptop bag with me at all times, especially at SEO conferences. (hence the bag on my cartoon version of me).
But technically, I never have the shoulder strap over on my right shoulder. Yes, I am a right handed man, but ever since some time in college, I stopped using my right shoulder to hold my bag.
One Friday I was playing tackle football in the park. I must have been about 19 or 20 years old. I intercepted a bad pass by the quarterback and began to run it back. This incredibly strong guy picked me up from behind, lifted me in the air, flipped me around so that I was upside down and slammed me down. I landed on my right shoulder.
At first, I thought I dislocated it. So I had a bunch of the guys hold me, and an other group of guys pull my arm the other way - trying to get them to pop it back in. It wouldn't go. But it was for sure out of the socket, I felt my shoulder and it was not connected fully to the arm.
After that didn't work, one of the guys drove me to a local EMT who tried to pop it back in. He failed and told me to go to the hospital.
So I went home, asked my dad to take me to the hospital (this is about 30 minutes later). My dad needed finish a few things, and then he took me (felt like forever). We got there, did an xray and they told me it was a separated shoulder and not a dislocated shoulder.
A separated shoulder is also known as a acromioclavicular separation or AC separation. For now, all I could do it take meds to relax the pain and try to keep it still, so it won't hurt so much. After that, they told me, it will heal, but never go back in place. If I wanted, I can go get cosmetic surgery, but typically, there is no need to.
Here is a picture from WebMD:

Separated AC stands for separating the "collarbone (clavicle) and the end of the shoulder blade (acromion)."
The Wikipedia has a xray of what that looks like, I can't find mine...

In any event, it took a few months to heal, my shoulder is still separated, I never had the surgery, but I typically do not have much pain there these days.
Since then, I had to learn to switch from carrying my bag on my right shoulder to my left shoulder. Took a while to get used to, but I did. Now, I wish I can switch back to my right shoulder, but it just feels to awkward.