Probably unpronounceable for a person may be pronounceable for a machine. So, it's kind of subjective.
An example can be a long chain of consonants like these:
- bcdfgdfgdddbcccdhlllllkkkljmnpqppqsttttvvbsvcxvzvxxvgk
- pqppqsttttvvbsvcxvzvxxvgjklllkkjjklkjvcxxczzzkzlzzvcpqppqsttttvvbsvcxvzvxxvg
- gbxcbbbzvxbcnnpqppqsttttsvcxvzvxxvgmklpqqqpqppqsttttvvbsvcxvzvxxvgvpqppqsttttvvbsvcxvzvxxvgjk.