The stationery of the hotel where Ken and Ray are staying has the hotel name as "De Rozenkransje - Brugge". Brugge being the Flemish name for the town of Bruges. Even a fictitious Belgian hotel would never be named like that, because the article is incorrect. 'Rozenkrans', meaning Rosary, would indeed have the article 'de'. However, 'Rozenkransje' is the diminutive and as such would always have 'Het' as the article. Even for proficient but non-native Flemish/Dutch speakers, this is a commonly made mistake.
When Ray tells Ken that they are in the Basilica of the Holy Blood, named after a phial of Christ's blood brought back from the Crusades, they are not. They are in the Jeruzalemkerk, a private church built in the 15th century as a mausoleum for a wealthy merchant's family. The Basilica is the most visited church in Bruges.
(at around 8 minutes) When the fat tourists are making their way up the tower one of the main characters refers to the tourists as "Americans". However, when the "Americans" yell at him telling him he is mean saying "screw you"; they have an Irish accent.
When Ray is preparing for his date with Chloe and is checking himself in the mirror, he is trying to decide whether to leave his top button open or button it. He decides to button it, then turns to Ken for approval and the camera cuts to a different angle. You can see in his reflection in the mirror that his top button is open. When he faces the mirror again and the screen cuts back, it is buttoned again.
When Ken is drinking alone in the bar, the bartender serves him a glass of beer that is well below full. The camera angle changes and then changes back again to show Ken is about to drink from a full glass.
In the park, Ken's silencer is for a .22 pistol (as evidenced by the small exit hole at the end), but the pistol itself is a larger caliber, 9mm or similar. The silencer would be destroyed by the first shot.
During the flashback scene of the Hit Gone Wrong, after the priest falls dead to the floor, we are presented the shot of the young boy, praying at the altar, with the gunshot wound in his head. Based on where Ray is standing (to the right of where the boy was), the wound on the boy's head is impossible. The hole is directly in the middle of his forehead, with blood spattered on his note, meaning that Ray would have to have been directly behind him to cause this injury. However, since he approaches from the side, this means that the bullets would be traveling perpendicular to the boy's wound, rendering the entire scene impossible to have occurred this way.
When Ken stops Ray from shooting himself in the head while sitting on the park bench, it is evident that the gun Ray is using was the one taken from Eirik the previous night. However, while this gun was originally loaded with blanks, Ray found some live bullets in Eirik's apartment (and even if he had shot himself with the blanks, it would have still been fatal.)