The last time I replaced my chain I made it too short by about an inch. I don’t remember exactly why — I probably just forgot to add the extra inch in when I wrapped the chain around the big ring and the big cog because I was busy messing with a camera also.
The result is a chain that’s too short. This can be bad, but you’re never supposed to “cross the chain” — that is, you shouldn’t ever shift to the big ring and the big cog at the same time. I figured I was safe because I don’t cross the chain.
Well, guess what I did last night? Yep, I crossed the chain and pulled my rear derailleur pulley to its limit until it cried “uncle” and refused to budge anymore after that. I broke the chain with my chain breaker tool and managed to get going again, but I now need a replacement chain because it’s too short by two inches now.
Be sure your chain is the right length. Too long, by the way, results in poor shifting.
Determining chain length isn’t rocket science — just wrap the chain around big and big and add an inch. Park Tool describes this process in detail along with a couple of other methods.