I go onto 1.8.7 and i can go on servers. i go onto 1.7.2 and it crashes

Put the summary of the bug you're having here

What I expected to happen was...:
Describe what you thought should happen here.
It was meant to allow me to go onto a server because it worked yesterday (13/06/2015)

What actually happened was...:
Describe what happened here
I go onto a server,tries to load up then goes down and tells me there was a crash.

Steps to Reproduce:
1. Put a step by step guide on how to trigger the bug here
2. go onto 1.7.2 and press play
3. Then click a server that is that version and a crash report should pop up.