"United miserable service again," Frist, 61, said in one of six posts on his Twitter account, USA Today reported.
"SanFran sold me and confirmed a seat then took it away," Frist said.
He later commented, "I booked a first class seat and now I'm in the very last row next to a bathroom that's in heavy rotation," and "I can confirm the toilets on this United flight work! My seat vibrates with each passenger's visit."
Frist later suggested the thin bathroom walls were likely a fuel-saving measure.
After United Airways returned a Twitter comment thanking Frist for flying and reminding him seat assignments were not guaranteed, Frist added: "Connection made; all is well. But do you refund passengers for selling seats that don't exist?"