Popup blocking has been a bit of an arms race. Browsers have added more and more advanced heuristics to distinguish "wanted" and "unwanted" popups, and popup script authors have worked hard to find useful exceptions to the browser rules.
Most of the holes popup users have found in the previous popup blocking approaches have been fixed in the most recent browser versions. Opera's popup blocking has been vulnerable to Flash-trickery but that will be history in Opera 9.
However, some users have been curious why the page http://www.popupcheck.com/freescan/popup/popup_test_standard.asp appears to show several popups that evade the blocker. Analysing this issue throws light on some interesting quirks and browser differences.
There is one immediately obvious aspect of a good popup blocker: it should be good at separating "wanted" popups from "unwanted". Opera fails this popup test because you did ask for the popups – you clicked a link to start the test. In other words, Opera is better at detecting what popups you do want than whatever browser this test was tested with.
Another and more technical aspect is whether the script should be able to detect that the popup it tried to open was blocked. Opera goes further to avoid "popup blocker detection" than the others do: we return a dummy window object to the script. A popup blocking return object demo shows that most other popup blockers simply return null. Hence, even if this test started as an "unwanted" action it would be unable to detect that the popups were blocked.
The test simply makes too many assumptions about the popup blocker(s) it will be tested against.