I'm not sure whether this is intended, but the spoiler begin to function differently after about five uses in the same thread. The first five tags appear blacked out entirely, with normal mouse highlighting required to make the text visible. Every tag after the first five seems to automatically make the text visible by simply hovering the mouse over it.
This thread is an example. Beginning at Kaolian's post every spoiler tag is automatically removed when I hover my mouse over it. I am browsing with Firefox 3.
This seems like a potential annoyance for people who are simply navigating through a thread. A minor issue for your already overloaded programmers to take a look at.