Elevators have a stop-between-floors button.

Many old elevators used to have such a button, but very sudden emergency stops can damage the equipment, and there was almost no situation where it would be useful to stop between floors. Mostly the buttons existed to give the passengers a sense that they were in control of their situation. Instead, for many decades now, for service personnel, elevators have had a key-operated switch that allows the elevator car to remain locked in position at a specific floor.

This is a very convenient device, allowing characters to be able to talk for an extended period of time, or to temporarily detain other characters. The elevator's emergency stop is always very gentle too.

Gold bars aren't nearly as heavy as one would think.

Actors pretend to struggle with gold-painted gold bars that obviously don't weigh nearly as much as a real gold bar would. Even if the prop bars are made of heavy concrete, real gold would weigh more than 8 times as much as that. A bar the size of a US quart of milk would weigh 40 pounds. A bar the size of a litre would weigh 19.3 kg.

Unless the actors have arms like Arnold Schwarzenegger, most of them would be unable to perform the role.

Police stick a knife tip into clear-wrap packages and eat the powder to confirm that it is heroin and not strychnine.

Anyone with enough education and intelligence to become a detective would not test an unknown chemical by tasting it.

This action has become such a cliché that audiences would think something is wrong if the detective didn't do it.

When asked to look through a microscope or telescope, people always change the focus and position of the instrument before looking.

Microscopes and telescopes must be very carefully positioned so that the object to be seen is directly in the center of view. Then they must be carefully focussed so that the object can be seen clearly. Beginning by touching the focus mechanism before looking means that not only won't the object be in focus, the object won't even be in the field of view. And the characters do this before they even know what they are supposed to be looking at, having just been told Look at this!.

Bending and carefully looking without touching is a little awkward and a lot boring. Touching and adjusting, even if it is before looking, demonstrates to the audience that the characters are skilled and know what they are doing.

Pulling a fire alarm activates the sprinkler system.

Pulling a fire alarm activates the ringing, and might also signal the fire department. Each sprinkler head is activated individually, and then only by becoming too hot, not by the alarm.

If a scene calls for a lot of sprinklers to be activated, there isn't any realistic way of making that happen other than holding a flame up to each one in turn. Who cares that it wouldn't happen in real life; it's a movie.