I finally discovered something that will make Opera crash pretty consistently. And what is the mystery web site? Gmail. Sigh. I'm actually going to test this out a little further on a couple of different platforms and with different accounts but this is the gist:
1. Log into web interface for Gmail
2. Open spam folder
3. Choose 'delete all spam' option
4. Pop. Opera crashes without comment, at least on OS X.
This is labeled 'Don't forget' so that my tired ass doesn't forget to test it out on a few other machines and file the appropriate bugs in the appropriate places. It is bafflingly consistent, though, which is a little bit of a concern for me since I lean pretty heavily on whichever browser I'm using at the moment.