I have heard a few times 꽃잎 being pronounced as 꼰잎. For instances that I can actually remember, Eunji of Apink sang it this way in Hopefully Sky, and Taeyeon of Girls' Generation did the same in Why.
I am aware of other sound changes, such as aspiration of consonants adjacent to ㅎ, nasalization of final consonants following a nasal initial consonant, adjacent ㄴ and ㄹ in any order becoming ㄹㄹ, ㄹ becoming ㄴ following a velar consonant ㅇ or ㄱ (which itself becomes ㅇ), and final alveolar stops becoming alveolo-palatal affricates before /i/. However, among these I don't see any rule that applies to 꽃잎. What rule does it follow, or is it an exception?