Critical Research Failure: In Personal, Reacher tells us that adding 20% to a body's speed adds 400% to its kinetic energy. That should be 44% - good but not quite as impressive. Particularly jarring as Child consistently portrays Reacher as a mental arithmetic whiz.
Usually when Reacher knows he's rescued everybody and everyone else is hugging their loved ones.
Played with when Reacher travels to California in Never Go Backto see about a possible teenage daughter he never knew of. The brief moments he encounters and chats with teenage girl Sam Dayton about stuff suggests they might hit it off as father and daughter, except it turns out Sam's not his. There's a hint of regret in the narration when Reacher drives away.
Padding: Child can get pretty obsessive about cataloging every single facet of a character's appearance, or a room, regardless of anything important to the story.
Though this may also be a bit of Fridge Brilliance, as that kind of excessive detail would be exactly what Reacher was noticing. Also, many elements are written as if the audience has never seen them before, which could be true of Reacher due to him having relatively little experience with Civilian life.
Tear Jerker: Reacher, when he finds out the cop and the witness he's befriended in 61 Hours have been murdered. It's one of the preciously rare moments where Reacher genuinely feels despair.
When Reacher learns that a murder victim was his brother. It's worse since his corpse was found with nearly every bone broken and a very large hole through his eye.
They Wasted a Perfectly Good Character: Lamarr of The Visitor was unattractive, uncharismatic, and had no attraction or chemistry with Reacher. She turned out to be a sociopathic antagonist. Some readers expressed that they would have rather seen her as an atypical Girl of the Week to Reacher and avoided the typical Romantic Plot Tumor that often plagues the book, but instead she's replaced by Lisa Harper, who's not much different or memorable than any other Girl of the Week.
A bit of Meta-Alternate Interpretation. The book Jack Reacher swaggers around because he's a huge mountain of muscle. By making that character played by Tom Cruise, it comes off more as Jack Reacher is The Napoleon.
Might be the only time you'll hear someone in a modern film actually say, with no hint of irony, "I'm not a hero. I'm a drifter with nothing to lose."
Tom Cruise for most of the part can put the Ability over Appearance but when a receptionist paints him as the guy who can kill a woman with his bare hands while hinting it is obvious by the looks of him, it's kind of ridiculous given Tom Cruise isn't exactly visually a powerhouse.
One-Scene Wonder: Robert Duvall as the one guy in the world who can converse on Reacher's level, though technically he appears in at least three scenes.
Werner Herzog only has a couple of scenes as the main villain but, being Werner Herzog, he makes one hell of an impact.
Tainted by the Preview: Fans of the books were livid about Tom Cruise's casting, as he looks absolutely nothing like the Reacher of the books (most notably, Reacher is huge, and his size is often an important part of his characterization).