Circular Drive

Alien Invasion. Natural Disaster. Kaiju attack. Big catastrophes naturally require a big response, and what better way to depict a big response then to show them mobilizing an entire army? There's only one problem: you can only afford twenty extras, two jeeps and three trucks... and one of those won't start.

Thus was born one of the classic techiques of cinema: The circular drive. Need to depict an entire armored division with only three tanks? simply drive them in a big circle that passes the camera and hope that nobody notices the markings don't change. You can turn a Suspiciously Small Army of twenty extras into an entire battalion if you frame your shot right. Note that this is not restricted to vehicles - pretty much any object will do, provided it can be steered appropriately. Takes advantage of the time-worn technique of having vehicles turn in front of the camera to add action to a scene, but the same three or four vehicles passing in the same repetitive sequence is going to be a dead giveaway. Alternatively, the same vehicle (or vehicles) may be noticeably present in multiple scenes for no apparent reason, which is particularly egregious when the scenes are supposed to be separated by time or distance.

Pretty much a Dead Horse Trope these days unless deliberately played for laughs. The modern equivalent is repeating the same airplane, boat or whatever multiple times using CGI. For a similar technique used in animation, see Wraparound Background.

Examples of Circular Drive include:

Film

Airplane!: Played for laughs, of course: The circle is obvious, the film is sped up, and as it goes on more and more incongruous vehicles (including a beer truck and a farm tractor) get added in.
Godzilla particularly noticeable with the firetruck scene: There's only one truck!
Top Secret played for laughs when the camera cranes back to display the entire circle.
- There is also a running feet example similar to the Hogan's Heroes example below... only to have the feet break into a time step on the third pass.
Raiders of the Lost Ark The truck blown up in Cairo is the same one used for the chase scene 20 minutes later. Especially noticable on repeat viewings.
Flyboys: The same generic, red CGI Fokker Dr.1 triplane appears muliple times, often within the same scene.
The famous long dolly shot that opens Akira Kurosawa's Rashomon was filmed by having the actor walk a figure eight path that crossed the dolly tracks twice. Clever framing makes it look like the camera is following the woodcutter through the forest but he's actually walking around it.
Lampshaded in The Truman Show, when Truman realizes this is happening with the vehicles that are passing by.
Gettysburg uses this technique to allow an admittedly large number of extras to represent two huge armies. Particularly noticable with the cavalry. The actors can change their uniforms but their horses can't change their coats.
In-movie variation in Help! - in the Bahamas, the police chief welcomes the Scotland Yard detective and presents a squad of policemen for inspection - the squad consists of only four men, the last in the row ducking and heading to the front of the row as the other two pass by.

Literature

In the Sword of Truth, Jagang did this to make it look like the army went one way, forced a confrontation with Kahlan as she went scouting, and pretended to believe that his ruse had fooled her. His army attacked that night.

Live Action Television

Used twice in the opening credit sequence of Hogan's Heroes with running prisoners and guards falling out for a midnight rollcall. The camera is aimed at their feet to hide the fact that it's the same men both times.
Mash: used in at least one of the "bugout" episodes, with anachronistic vehicles thrown in to boot.
An episode of Land of the Giants featured the heroes stealing a bunch of giant spark plugs. There was only one spark plug prop, though clever editing handles the issue quite well as they steal one at time and immediately hide them all.
An in-Universe example from Dad's Army where the platoon was on guard at a local Italian POW camp and had to conceal the fact that most of the prisoners had temporarily escaped (they were working for Private Walker and would be back before dawn). They got the remaining ones to jog through their hut over and over again while the authorities did a headcount.

Tabletop Games

The intelligent raptors in a Wild West Dinosaurs D20 campaign setting are said to do precisely this to show larger numbers than they actually have.

Real Life

During The American Civil War's Siege of Yorktown in 1862, Confederate general John Magruder actually did this to convince Union general George McClellan that Magruder's small force was a big one. It worked, and McClellan settled in for a siege rather than overwhelming the vastly outnumbered Confederate garrison.
This was a huge part of the traditional Victory Day military parade in the Soviet Union. Planes, for example, would fly over Red Square, fly out of the Moscow airspace, and then loop around for another flyover. Furthermore, there was a very large unit of the Soviet military devoted solely to parading on important national days.
Circular Drive -style deceptions were popular with the western Allies in WWII, originating with the British in the Western Desert and later embraced wholeheartedly by the U.S. Army and climaxing in an entire false invasion force off of Calais on D-Day. Literal CircularDrives were a popular part of these deceptions, with trucks full of mannequins and fake insignia.