When packing the bullets in the candy box, and when handing over the candy box to the assassin, Jack doesn't wear gloves. Given how careful he's shown to be, one would expect him to not leaving fingerprints, especially given that he knows the gun will most likely be left at the scene.
When Jack and Mathilde go to the river to test the new rifle, she asks, "Muzzle velocity?" Jack answers, "About 360 miles an hour. That's including about 20 miles an hour off with the sound suppression." A ballistics-savvy person like Jack would have answered in "feet (or, meters) per second."
Woman testing Ruger Mini-14 shoots. Misses low. Corrects for elevation with 4 to 5 clicks right. First error is that generally right turns on the elevation turret push bullet impact lower. Her next shots were higher. Wrong elevation correction. Second error is that if her turret is in 1/4 or even 1/2 MOA per click, she only adjusted 1 to 2" at 100 yards. She was shooting from approximately 50 paces (in heels over uneven ground), or 100 to 150 feet, or probably about 40 yards meaning her elevation correction would have only changed the POI (point of impact) by about 1" maximum. The target plate was nearly 10" in diameter. Her POI shift from first shot to the three shot group was over 6" higher. Likewise, granted it was off hand, assuming 40 yards, she shot a 11 minute, 3-shot group...horrible by any standard much less a sniper with a scoped rifle. That's barely a good group for a production handgun in the hands of an enthusiast.
When Jack and Clara go to the restaurant, they order still water and the waiter brings them a bottle of Aqua Panna. But as they talk and the camera angle changes, the bottle on the table becomes a green San Pellegrino bottle (sparkling). When the angle changes again, the bottle is again an Aqua Panna.
When Jack and Mathilde first meet outside the café, the scene has two shots: one facing Jack, the other facing Mathilde, switching between the two as they talk. When the camera is facing Jack, Mathilde has her sunglasses in her hand, but when the camera is facing Mathilde, the sunglasses are on the table. The position of the sunglasses on the table also shifts during the course of the scene.
When Mathilde first fires the rifle she shoots about a foot to the left of her metal sunflower target. To correct this she should have taken the SIDE cap off the telescopic sight and turned the 'windage' screw half a turn clockwise moving the point of impact to the right. Instead she removed the TOP cap and adjusted the 'elevation'. This adjusts the up and down point of impact. Her second burst should have still missed the target from the same aiming point.
When Jack and Mathilde first meet to discuss the rifle jack is to supply, she asks for a "silencer" jack tells her he can only provide a "suppressor".
A weapons expert/gunsmith like jack would know that there is no such thing as a silencer and it is just a slang term for a suppressor. Yet he talks as if they are two different devices.
Woman asks for "silencer". Clooney says "I can only make you a suppressor". A real, REAL expert would know that the terms are interchangeable. While enthusiasts will often correct someone who says "silencer" to say "it's a suppressor...it doesn't make it 'silent'", the inventor of the "silencer", Hiram Maxim, patented the "Maxim Silencer" in 1909. Interesting note, his dad invented the "Maxim machine gun." Likewise, when you buy a "silencer", the proper terminology for your Form 4 (the paperwork you file to buy one) requires that you call it a "silencer", not a "suppressor". In the industry, the CORRECT term is "silencer", although both are used interchangeably. However, no actual expert or industry professional would correct someone to say "I can't build you a silencer, only a suppressor"...unless they were playing games to avoid registration with the ATF (in America, anyway) and they'd still face felony charges if unlicensed or unregistered. Maxim also invented a car muffler and, coincidentally, "silencers" in many countries are called "mufflers".
After Jack assembles the rifle, he pulls back the bolt to cock the rifle and dry fire it. However with an empty magazine in the rifle, as it was, the bolt would lock in the rear position and not fully cock for dry firing.
Mathilde specifies a rifle with accuracy at 150-175 Meters (approx. 160-190 yards) and with "rapid" rate of fire. The rifle provided (Ruger mini-14) even heavily tuned with a scope is only accurate to around 100 yards (92 Meters). The rile is also semi-automatic (Cycles the next round automatically but only fires a single shot when the trigger is pulled) so the rate of fire is down to the shooter.
Jack/Edward cites the muzzle velocity of the rifle as 360 miles per hour (mph). No expert cites muzzle velocity in mph. It's always in feet per second (fps) or meters per second (mps). Beyond that, 360 mph is only 528 fps. That's hardly a reasonable muzzle velocity for that rifle, and it would likely be very inaccurate.
Jack, the gunsmith, makes a modification to the rifle that he sells to the assassin that later causes it fail in a manner that severely inures her and causes her to fall to her death. A trained professional would have not only inspected , but also cleaned their weapon prior to its use.
There are no "adjustments" which could be made to such a weapon that could cause such a major failure and not be detected by a shooter who took care of said weapon.
When Jack receives the Swedish newspaper clipping the text following the headline has several grammatical and punctuation errors indicative of it being written by someone whose native language is English. One of several examples from the article is that in Swedish homicide investigation is a single word - "mordutredning" - not two ("mord utredning").