h2g2 The Hitchhiker's Guide to the Galaxy: Earth Edition

Another classic one I've heard is the minigun in predator, uses more bullets in one scene than a man (even one as big as Jesse 'The Body' Ventura) could carry, to say nothing of the weight of the gun itself or - and possibly the most critical criticism - that he could fire, with any accuracy. The kick-back on one of those is massive - old Newton's 3rd law - but he just stands there and grunts, maybe breaks a sweat, These things tend to be bolted to the ground or the helicopter for a reason.

Most extravagant gun shots reactions that send the victim flying backwards make the same error - the guy shooting the gun is still standing. Robocop might be an exception cos he weighed more so had a great inertia to over-come.