In 1.4.2 there was no problems with the nether generation. However in 1.5.1 there are these random areas where lava is. is just a 1x1 area. There never was a problem with this in any other version. Hopefully this was not added. In the photo i used x-ray texture pack to show what i am talking about. both areas are taken at the same place in the nether and both are the same seed. I used the same texture pack also. I have tried this in the most current sanpshots. All of them have this problem as well as 1.5.1 and 1.5.2. I don't know whats going on. Sometimes there is twice as much lava areas in certain spots. It makes nether tunneling almost impossible to do. i have also looked on the wika and there is no info on the topic.