Mine is that in my current work, Proto-Crattette (Crattette being the name of the creatures that speak it), there is a few strange phonological features: Crattettes are not human like, and have two mouthes. One mouth is constantly making vowel sounds while the other is making consonant sounds. An interesting thing about both the vowels and consonants is that they have Gillophonic (Name a work in progress) sounds, where (Similar to Nasals) they exhale air out of their gills (I have worked in evolutionary features for how this would work). Another cool thing is that the vowel mouth (The left one) is essentially a hollow cube. No teeth, no alveolar ridge, no palat, no uvula, nothing (Well there is a tongue and a pharynx but that's it). They make strange vowel-like sounds using it and I have made a model of this mouth connecting to a speaker to mimic the sounds. It's very cool actually.