Most consonants in Hittite appear in two variants, conventionally called "voiced" and "voiceless": "voiceless" consonants are written twice in a row, while "voiced" consonants are written only once. For example, the word appa "later" is variously written a-ap-pa or ap-pa, but never *ap-a or *a-pa, while the word apa "that" is always ap-a or a-pa, not *ap-pa. So it's thought that the former was something like
/apa/, and the latter
/aba/. (Or maybe fortis-lenis, or short-long, or…)
However, this distinction usually only shows up intervocalically: initially, finally, or in clusters, the writing system has no consistent way to express it (as there are no glyphs for lone consonants).
So—is anything known about this contrast in other environments? Do we know if it was actually neutralized in other contexts, or if it just didn't show up in writing?
(We might know this based on, for example, the spelling of clusters: if a form like nahh-teni "you all are afraid" is consistently written *na-ha-te-ni, for example, that's decent evidence that the hh ended up getting voiced; if it's consistently written *na-ah-ha-te-ni, that's decent evidence that it remained voiceless. But I don't know if either of these writings actually happens.)
EDIT: As fdb pointed out, voiceless consonants aren't quite "written twice in a row", since there are no signs for individual consonants in Hittite cuneiform. Rather, two adjacent signs both indicate the same consonant: ap-pa is written with a sign for
/ap/ followed a sign for