In this other answer, TKR suggests that the Homeric dative οἱ might have once been something like *ϝϝοι, with initial long [wː]. This makes sense to me, etymologically, since it may have come from a PIE form starting with *sw-.
Vegawatcher also cites Benner about a related matter in Homer:
A short final vowel may make a long syllable when the next word begins with a liquid λ, μ, ν, ρ—or digamma, or sigma.
This irregular lengthening aside, though, it makes me wonder: do we ever see words that consistently start with a long consonant? In other words, are there words that start with a single consonant (rather than a cluster) that consistently make the previous syllable scan heavy, indicating that there's an underlying or historical *ϝϝ, *ρρ, etc? Or do there seem to have ever been words like that in the history of Greek, even if they were no longer consistently so in Homer?
