In a language I am studying I have just noticed a significant but subtle difference in the length of [f] segments in tonic versus atonic syllables (an ~50ms difference which is statistically significant). When mentioning the effect to a colleague, I was asked whether such an effect, where consonants differ in their lengths (closure or constriction time) depending on their position in the word, is cross-linguistically common. I said that surely it must be, but I realized that I could only think of two examples, in English [s] (Klatt 1974) and Ibibio nasals (Akinlabi & Urua 2002: 135, fn.11).
Can I have help with other examples?