I just learned that some Greek neuter nouns of the third declension with a nominative/accusative singular form ending in -ς have oblique stems in -τ-, which surprised me.
I expected τ-stem neuter nouns to have a nominative/accusative singular that simply drops the -τ-, and s-stem nouns to simply have loss of intervocalic s in the oblique forms (as in γέρας, γέραος/γέρως).
A Wiktionary appendix makes a vague but somewhat informative comment about the appearance of -τ- in -μα, -ματ- words and -r/-n- heteroclites (which seem to be fairly well represented in Greek; e.g. ἧπαρ, ἥπατος):
Interestingly, the τ in the stem is a common feature of Ancient Greek words derived from PIE neuter n stems, which is not well explained.
However, I haven't heard before of s/n heteroclites, so I'm not sure if this is related to the appearance of -t- in the oblique stems of neuter nouns ending in -s.
The original example I ran into was κέρας/κέρᾰτ-, from this discussion. It apparently was variable in declension.
A bunch of other examples I took from Wiktionary: οὖς, σταῖς (Wiktionary suggests "influence from" στέαρ), φῶς, ἅλας, δέρας, πέρας, τέρᾰς, ὄπεᾰς (Wiktionary suggests it replaced older *ὄπεαρ), κρέας (variable), (also πᾰ́γκρεᾰς), σέλᾰς (variable), ἐρῠσῐ́πελᾰς