So, you have an experimental piece of technology for which there is No Plans, No Prototype, No Backup, or is Imported Alien Phlebotinum. You're about to begin an important test, when somebody discovers a minor fluctuation in one of the readings. A higher-ranking scientist or military officer says that it won't affect the test. Except it almost always does. Sometimes it affects it to the point of a Tokyo Fireball. This is a side-effect of The Law of Conservation of Detail. Minor fluctuations happen all the time in complicated experiments, but unless it's going to directly affect the plot, you won't hear about it.
- In Spider-Man 3, the scientists running an experiment involving a particle accelerator and sand notice that the weight of the sand is greater than expected. They write it off as a bird that will fly away once the experiment starts. The "bird" is actually Flint Marko, about to be turned into The Sandman. One wonders what 200-pound bird the scientists were thinking of.
- In the book The Thor Conspiracy, a single signal is wrong on the test boards for a nuclear test launch being held in the South Pacific. The error causes the missile to go off inside the ozone layer, catching it on fire. Not a perfect version of the trope - the signal on the board was green at the time of launch, but had been red earlier, and they refused to let the technician who had noted it then halt the launch countdown for a systems test.
- Justified in an episode of Stargate SG-1: Sam noticed that the power level of a force shield surrounding the town they were in (protecting it from the poisonous wasteland outside) was dropping. The other scientist present insisted it was nothing. He was being brainwashed by the computer.
- Almost this exact phrase is uttered in the opening scenes of Half-Life; soon afterwards some rather unpleasant creatures make their debut.
"Uh, it's probably not a problem, probably, but I'm showing a small discrepancy in... well, no, it's well within acceptable bounds again. Sustaining sequence."
- It's subverted in Half-Life 2: Episode Two, when the project appears to work without a hitch despite the "eight and a half pound anomaly". Of course, the anomaly in question is Dr. Kleiner's pet, Lamarr, setting up a nice Chekhov's Gun for future installments, on the rare chance Valve remembers Half-Life exists and releases Episode 3.
- In the Interactive Fiction game Trinity, you have to sabotage the eponymous atomic bomb test in a way that will be written off as Within Parameters.
- Truth in Television. The inherent variability of the human body can mean that, in the results of medical testing, a slightly off-average result can still mean that a disorder is present. Because it usually means absolutely nothing, and responding to it generally does nothing but waste time and money (and sometimes hurts the patient as a side-effect of unnecessary tests), doctors have learned to ignore these anomalies—which works fine, except when there really is a big problem to worry about.
- Scientists know that the results of their experiments have a lot of variability, and have to train themselves to ignore what seem to be patterns but aren't—which means that, occasionally, they miss a pattern that really exists.
- The Chernobyl nuclear disaster happened as a result of this - the inexperienced in comparison control room crew had no idea of what was actually going on in the reactor, relying on instrumentation that claimed the water level and the power level was fine. The only problem was the readings were absolutely wrong, and correcting for them only worsened the problem - and by the time the instruments caught up to reflect the problem (loss of water coolant), the situation was absolutely irreversible. A second and even more tragic form also happened at Chernobyl: radiation monitoring systems such as dosimeters were overwhelmed by the amount of radiation, either malfunctioning or showing top normal readings - the end of their range, which convinced people they were safe on site as opposed to being lethally irradiated.
- At least two BP plant explosions (Texas City and the Macondo/Gulf well blowout) had this as a major feature - the readings seemed to be within normal limits, but actual conditions were far, far more dangerous - and combined with multiple Failsafe Failure, the result each time was literally explosive.
- An episode of Stargate Atlantis ("The Return") has a test which has a strange reading, but it really doesn't affect the test. It heralds something far more important. Which was probably to subvert audience expectations after an earlier episode featured readings "within parameters" that eventually blew up a solar system. Which was brought up several times later in-universe, much to Dr. McKay's ongoing chagrin.
- In Star Trek: First Contact, when the Phoenix is about to launch, a red light is on, which Zephram attempts to fix by the age old "hit it hard" method. When it stays lit he says "Ignore it". It turns out not to be an issue, and the flight is successful.