I have a sinking feeling that this might be bad. The plant is a melothria scabra (Mouse Melon/Mexican Sour Gherkin). My concern is that it might be a virus, however it doesn't quite resemble any photos I can find on the web.
It's affecting only a few newer leaves at the tip of one of the vines on one of my plants. There's a weird translucent quality to the lighter parts of the leaf--as though it weren't really a discoloration so much as that those parts are thinner. There doesn't seem to be deformation of leaf shape, though--although I guess I'll know that better when the two smaller leaves that are afflicted grow a bit. There have been some aphids in the garden, although I haven't noticed any on this plant. Aside from slug damage early on, the mouse melons have been basically pest-free so far.
One of my plants next to this one got some kind of wilt a couple weeks back, and I pulled it. But there were no symptoms other than catastrophic wilting--no color changes or anything.
Front of leaf (click for enlargeable photo)
Back of leaf (click for enlargeable photo)