<st:include "snippet.html">
It's generally a good idea to include the full error message when asking for help. In this case, it would probably tell you that your snippet isn't legal XML.
Try <j:include uri="something.html"/>; assuming the file is in the same dir. I only tried it in hpi:run though; so may fail properly deployed. It's not something I've done before (and cannot really think of a use case for either).