It doesn't reproduce on Meta, but it will on any beta site. To see an example, check out this meta.chess post:
Edit:
I noticed that on SciFi, English, Area 51, and Electrical Engineering (at least - those are the only ones I've tried), spoilers with monospace will show the monospace part whether or not the mouse is hovering over the block (though the normal font will continue to operate normally). On those sites, however, it hasn't yet joined the Dark Side.