So, I'm developing a website that uses a custom font via the Google webfonts system. In Firefox and Chrome all the fonts look fine. But in Opera 11.62 running on Ubuntu 11.10, 15px Lato looks really weird: https://i.sstatic.net/dsNJf.png . The font appears to vary in weight part way through letters. I think that its just applying hinting way too much. Google webfonts is sending the file in .woff format, if that affects anything.
How can I either fix this or prevent Opera from applying the custom font?
Yes, your screenshot definitely indicates that the font's hinting is not perfect. You could try running Lato through the Font Squirrel Generator and see if you get better results. Autohinting routines are a bit hit or miss, though ours is getting better.
