Crowd Hockey

So a character (usually the protagonist, occasionally one of the supporting characters) accidentally drops something (usually the MacGuffin, sometimes simply something of extreme sentimental value) in the middle of a crowd. Naturally, every time the character tries to grab it, it gets kicked away by someone in the crowd. Lather, rinse, repeat, until one of the inevitable conclusions:

Someone manages to get to it (either the character who dropped it or someone else who's looking for it),
It gets trampled on by someone, thus destroying it, or
It gets kicked into a hole where it is inaccessible through the crowd and thus displaced until some later scene, where someone retrieves it.

Occasionally, this occurs during a fight scene, and as such if someone manages to get ahold of it, it gets knocked out of their hand or otherwise gets incapacitated, in which case the scramble for the MacGuffin begins anew until one of the previous three results occur.

Related to Mobstacle Course—if the pursued loses the MacGuffin in a crowd while being chased, this happens. Alternately, after the MacGuffin is retrieved, the retriever may be chased through the crowd.

Examples of Crowd Hockey include:

Film

Indiana Jones and the Temple of Doom starts out with a shootout in Club Obi Wan over a diamond, which Willie attempts to retrieve. The panicking crowd kicks it all over the place, including into a pile of ice and also a vial of antidote Indy needed to retrieve.
In the Clint Eastwood film Hereafter, one of the characters loses his brother's baseball cap while people are queuing to get into the Underground, and it keeps him searching long enough to keep him off the train he was trying to board.
Batman and Robin take this quite literally in the opening battle with Mr. Freeze, when the latter is trying to steal a giant diamond and the heroes and henchman play actual hockey (complete with sticks and skates) to get it back.
In Pirates of the Caribbean: The Curse of the Black Pearl, Ragetti's wooden eye falls out during a battle on deck, and he scrambles to grab it. The battle ends about the same moment he finds it again.
In Teenage Mutant Ninja Turtles II: The Secret of the Ooze, a fight in the TGRI laboratories turns into Crowd Football, with the Turtles trying their best to keep a canister of mutagen from falling into Foot hands.
Rush Hour 2 does this with the detonator in the Casino.

Live Action TV

Chuck: "Chuck Versus the Cubic Z" has Morgan chasing after the eponymous Cubic Zirconium during a BuyMore riot. He doesn't find it.
An episode of Psych concerning a potential plague had this happen to a vial containing the disease. It was kicked around a bus station until Shawn managed to catch it before it shattered.
Smallville: In the episode Fortune, Lois is trying to find her engagement ring on the floor of the Fortune casino, which is being kicked around the floor by the evacuating patrons.