Was the Taz, R. Davi Ha-Levi (1586-1667), a Sabbatean? Many -- including R. Ya'akov Sasportas and R. Ya'akov Emden -- relate the story of how in the summer of 1666, the Taz's son and son-in-law went to meet with Shabsai Tzvi, the false messiah, and investigate his claims. By all accounts, they were very impressed and returned to Poland with high praise for him. They also brought with them a "cure" for their elderly and ailing father/father-in-law.
Keep in mind that this was less than 20 years after the pogroms of 1648 (Tah ve-Tat) and there was a high messianic fervor in the air. Also note that the excesses of Shabsai Tzvi were not necessarily known to the Polish rabbis.
Nevertheless, while Gershom Scholem claims that the Taz was also a Sabbatean, R. Elijah J. Schochet, in his 1979 biography of the Taz, disputes this. He reviews all of the evidence and concludes (p. 23):
[T]here can be little doubt that Rabbi David's son and stepson were impressed with Shabbetai and convinced of the authenticity of his messianic visions. There is, however, no real evidence to back the claim that Rabbi David himself was a believer in Shabbetai Zevi.