One blurry area is in segments variously called semivowels, semiconsonants, or glides. On one side, there are vowel-like segments that are not in themselves syllabic, but form diphthongs as part of the syllable nucleus, as the i in English boil . On the other, there are approximants that behave like consonants in forming onsets, but are articulated very much like vowels, as the y in English yes .
A confusing aspect is the classification of segments called semivowels, semiconsonants, or glides. One group consists of vowel-like sounds that aren't syllabic themselves but contribute to diphthongs within syllables, like the "i" in "boil." The other group includes approximant sounds that act like consonants when forming initial sounds but are articulated more like vowels, such as the "y" in "yes."