Andrew Gregory points out that some browser detection scripts might have trouble when Opera 10 eventually rolls around. (Edit: Hallvord also comments.) Why? Because one of the easiest, ways of testing for a version number is to do look for the the “Browser n” or “Browser/n” patterns. The problem is that this strategy only grabs the first digit of the version number. That works fine for 1–9, but once you hit 10, suddenly it looks like 1 again.
Firefox and Safari, currently at just before and just after 3, are likely safe for now, but IE is creeping up on 8, and with their new, faster release schedule, IE10 may only be a couple of years away.
I’ll admit, I’ve written code like that myself (not the specific example, but I’ve done regexp matches that only look at the first digit), but always on sites that I expect to be able to maintain. Of course, one of the lessons to learn from Y2K is that shortcuts get entrenched, and code you thought you’d have time to clean up long before it became a problem has a tendency to stay in use far longer than you expected. And we’ve seen the same thing with web script archives, where someone’s example code that mostly worked in IE4 gets enshrined as “the” way to accomplish something, even though there have been better ways that work more consistently for years.