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.
There have been several occasions where Reacher has described probability of anything as "50/50", due to the belief that something either happens or it doesn't. Anyone with a basic understanding of statistics or probability can tell you that this is very wrong.
"Funny Aneurysm" Moment: In the second book, the main FBI character, while telling his boss about two men helping him, lists their not complaining about the FBI salaries as among their best qualities and is jokingly asked if he can clone them. Later, Reacher theorizes there is a corrupt FBI agent and asks Holly which ones don't complain about the salaries, or rather which ones have enough money from potentialy illegals sources that they don't need to complain about the salaries anymore.
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.
He's Just Hiding!: in Bad Luck an Trouble Sanchez is the only member of Reacher's old unit captured by the Big Bad whose body isn't found, with some hope being held out throughout the book, with the possibility of his survival even being used to lure characters into a trap. This becomes Ascended Fanon when added into one of the ambiguously cannon Hunt For Jack Reacher novels by Diane Capri, although it also gives him a FaceHeel Turn and feels very unfaithful to the characters in general.
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.
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.
They Wasted a Perfectly Good Plot: Often, The Reveal about what's going on doesn't live up to all of Reacher's intense speculation about it throughout the book, with Nothing To Lose, being a major example, with the teasing of multiple factions in the Town with a Dark Secret ultimately not reaching its potential. The Killing Floor with a witness describing early on how the criminal conspiracy has ten members nearby might have provided more of a challenge out of identifying those ten hadn't been Obviously Evil, while others weren't even named or described that well, and two were only seen from a distance briefly, opening and closing a gate, before Reacher fought them.
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).