This is impossible
 

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So yesterday I installed this new derailleur. This morning I went to get the bike to go for a ride, I thought Id check the derailleur (no idea why). This is what I found, what happened here is the chain somehow got to the other side of that little metal bit (instead of comming off one jockey wheel to the other one). Physically I think this is impossible, it is driving me crazy because I can see no possible way that this happened. I spun the pedals and it makes a awful noise now.

All things are possible with Jesus.
I did that once. Did a hasty job of threading a chain through the pulleys and went on the wrong side of the guard.

How long ago did you replace this chain? Any chance of passing it back through at the narrow point of the chain?

What kind of derailleur & chain is this? Sram? Because it is indeed impossible (with Shimano.)

If you installed a new derailleur, you had to thread your chain through it. When you did, you must've run the chain on the wrong side of the little metal tab. It's a common mistake. Hopefully you're using a chain with a quick link (such as KMC, SRAM, or Wippermann) so you can break the chain, re-route, and reconnect in a minute or so.

Yup, I did that once. Heard the extra noise and stopped to see that I was a bonehead.

You need to get in your mechanic's face.

It's only impossible if you assume you had it right last night and it somehow moved. So, you can safely rule that out and be left with the fact that you installed it wrong, which as pointed out above is pretty common, and simply didn't notice at the time.

BTW - you don't have to cut and splice he chain to fix this. You can slightly loosen the upper bolt, then remove the lower pulley entirely. That will allow you to scissor open the cage, free the chain and route it correctly when you reinstall the lower pulley.

Yup, I did that before, too.

I think you know why. Maybe only subconsciously though. Haha

Been there, done that...

Elf on the shelf is up to no good.

I have an uneasy feeling about this. If it was okay when you installed it...

...then someone close to you is a ... saboteur!

Trust no one.