Innocently browsing today, I encountered this HTML comment that gets rendered due to <-- instead of <!-- ... I couldn't even find the template that caused it, and leave it in your hands, as I've got to go, as Dana Dane said.
$ w3m -dump http://en.wikipedia.org/wiki/Flatworm |head Flatworm
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Jump to: navigation, search Good article Flatworm!<-- This template has to be "warmed up" before it can be used, for some reason -->
Platyhelminth worms Fossil range: 40–0 Ma^[1]