Several chunks in an area I frequently visit randomly reset to Deep Ocean after updating to 1.7.4 from 1.7.2. Not bothered by this, I decided to take advantage of the situation by building fences and paths in the lone village that had spawned there. Upon loading the chunks by either saving and quitting or leaving the area and returning, any modifications I had made were reset as if I had made none.
As shown in the screenshot, this affects a massive area. I was aware of the terrain generation issues as warned by previous updates so I had visited this area a lot of times to ensure it would not be ruined.
I have seen this is an issue in multiplayer, but I had not seen an instance of this in single player. I am running vanilla Minecraft 1.7.4, world generated in 1.6.4, with 64 bit Java 7u45. I have not used any mods or external tools of any kind if that helps.