In the Star Trek: Deep Space 9 episode, "In Purgatory's Shadow Part-1", Worf and Garak are captured and sent to a Dominion prison only to discover that Bashir is already there. Bashir is wearing an older style of uniform which means he had been in the Dominion prison for several episodes. Which means the Bashir on Deep Space Nine space station is a Changeling impostor.
GARAK: When were you brought here?
BASHIR: Over a month ago. I was attending a burn treatment conference on Meezan Four. I went to bed one night and woke up here.
In the episode, Rapture, Bashir operates on Captain Sisko's brain. We learn in a later episode that this was the Changeling Bashir and not the real doctor doing brain surgery.
When the crew of Deep Space 9 learn that Bashir was replaced by a changeling and operated on Captain Sisko, did they check if the impostor futzed with Sisko's brain?
The changeling could have placed an electronic device into Sisko's brain, subtly damaged key brain sections, or rewired the brain to make Sisko more susceptible to suggestion. I am surprised the changeling did not outright kill Sisko on the operating table. It would have looked like a normal death and the Federation would have lost a key tactical leader.