Minecraft 1.7

I play on servers and worlds in 1.7 occasionally both modded and un-modded. However, whenever I join a server or world, it crashes. I'm not sure if many other people are having this Issue but I would like to know exactly what the problem is. I would also seem that 1.7 doesn't seem to be available under the affects version/s, so I apologize if this is put under the wrong area.