Within Parameters

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.

Film

 * 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.
 * It's the Marvel Universe, home to the scientist who shot himself up with a rage-inducing Super Serum and the guy who tests flying Powered Armor suits by putting them on and forcibly slamming himself into concrete walls. Rigorous safety standards don't appear to exist there.

Literature

 * 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.

Live-Action TV

 * Far, far too many TV series dealing with experimental technology.
 * 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) were dropping. The other scientist present insisted it was nothing.

Video Games
""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.""
 * Almost this exact phrase is uttered in the opening scenes of Half Life; soon afterwards some rather unpleasant creatures make their debut.


 * It's subverted in Episode Two, when the appears to work without a hitch despite the "eight and a half pound anomaly". Of course, the anomaly in question is, 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.

Real Life

 * 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.
 * Similarly, 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.

Live-Action TV

 * 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. Mackay's ongoing chagrin.