The spec establishes that intraword underscore does not indicate emphasis, but permits some curious behavior when used in conjunction with asterisk. I ran into this writing up mathematical terms, e.g.
*t*_1 + *t*_*a*
using underscore to indicate subscript, though not actually formatting it as such. (I’m not using any extensions to permit actual sub/superscript formatting.) Most Markdown parsers interpret _ as establishing emphasis in this case, i.e.
<em>t</em><em>1 + <em>t</em></em><em>a</em>
presumably because they see the *s as punctuation, absolving the first _ of being part of a right-flanking delimiter run and similarly for the later. I would argue, however, that the asterisks aren’t really there as they’re consumed by parsing; allowing them to serve as punctuation seems particularly strange.