Bashir surmises that Sloan's plan is to kill Koval by making it appear as though he has an accelerated case of Tuvan Syndrome, and that the only way to make this happen is to maneuver Koval near a phaser array and surreptitiously irradiate him. Romulans don't have phaser arrays because they don't use phasers. Romulans use disruptors.
Sloan suggests obtaining skin cell samples from Koval to analyze for signs of Tuvan Syndrome. Bashir agrees that this would work. Tuvan Syndrome is said to be a neurological disease, similar to Amyotrophic lateral sclerosis (aka Lou Gehrig's Disease). Neurological disorders cannot be detected through skin cell analysis, as they do not affect the skin. Physical tests of the afflicted person's nervous system must be conducted.
In the last external view of the USS Bellerophon, the designation NCC-74656 can be seen on the hull. NCC-74656 is the designation for the title ship from Star Trek: Voyager (1995), from which the stock footage came.
In the establishing shot of ships in orbit around Romulus, several of the Romulan warbirds are the exact same ship viewed from the exact same angle with the exact same lighting. The production must have used the same shot of one warbird model and pasted it repeatedly into the scene at different sizes to show depth of field.
As Bashir and Sloan are reviewing the various Romulan holographic players in Sloan's quarters, Koval's eyes can be seen to blink in various shots. If this were truly a hologram, the "character" would be inanimate and nothing more than a projection. The actor playing Koval is obviously standing in the same scene as the two primary actors.
Dr. Bashir's presentation of the disease known as The Quickening features two photos of the same patient at different stages of the disease, but both are the same photo; the only difference is the severity of the lesions on the face of the patient. If they were actual before-and-after photos, there would subtle differences in lighting, angle, background, and rate of exposure.
Before Admiral Ross and Dr. Bashir have their "off the record" conversation in the Bellerophon's Ready Room, the establishing shot of the ship shows it moving at impulse. The starfield visible out of the ready room's windows does not move during the entire scene.
When Bashir enters the replimat with Garak, he goes to the replicator and retrieves a beverage without ordering it.
Sloan tells Bashir that Koval being seated on the Romulan Continuing Committee would be disastrous for the Federation because Koval would push to negotiate a Romulan peace treaty with the Dominion and the Federation/Klingon forces would be on their own. The Romulans became embroiled in the war against the Dominion because of (falsified) intelligence that showed the Dominion planned on invading Romulus. No Romulan government would pull out of the war effort and try to negotiate peace under those circumstances.
Sloan informs Dr. Bashir that the Tal Shiar is handling security for the conference on Romulus. The Tal Shiar is the Romulan secret police and intelligence branch of the military. Essentially, they are a group of spies. Even if they were involved in the conference, they would not be handling security. The regular Romulan military would do that, with the Tal Shiar gathering intelligence from the shadows.
Ross says he will have Sloan's access at the conference limited to a few low-level lectures, yet Sloan is present at Bashir's lectures on biogenic weapons, a high-level topic.
Ross, Bashir, and Cretak enjoy a glass of Romulan ale at a mixer onboard the USS Bellerophon. Romulan ale had previously been illegal in the Federation not just because of a trade embargo with the Romulans but because it is highly intoxicating to a degree many species would find dangerous. Even if it is now legal to consume, drinking such a beverage at an official function would be highly unprofessional.
Admiral Ross tells Bashir that Sloan "was supposed to be beamed away a split second before the phaser beam hit him." Romulans don't use phasers. They use disruptors. A lifelong military man like Ross wouldn't make such a mistake.
Admiral Ross states that he will have Sloan confined to quarters. That would immediately give away that Ross is onto him and that Bashir is working as a double agent.
When testifying before the Romulan Continuing Committee, Bashir says that the plot to assassinate Koval was masterminded by a man "known only to me as 'Sloan.'" In Sloan's previous appearance, he gave his first name to Bashir as 'Luther.' There is no reason for Bashir to think Luther is any more or less of a false part of an identity than Sloan.
Sloan is meant to be a spymaster, yet he gives a speech to Julian about how he is a "man of secrets" after opening the door to Julian's quarters. Anyone passing by could have heard him. Additionally, he then walks out of the quarters (and is subsequently not seen, presumably having transported away). If someone had been passing by, they'd have seen him easily. Not very good spy work.
After leading Bashir into an interrogation room Koval says, "Now we'll have our private talk." Yet two Romulan guards remain in the room with them and assist in the interrogation.