Slippery Soap

So you have a character in the shower (or bath) and they reach for a bar of soap. The soap proceeds to be very uncooperative, slipping out of the person's hands. Occasionally, it happens when someone is washing his (or her) hands. Often seen as part of a Black Comedy Prison Rape scene, or any time a Shower Scene gets Played for Laughs, really.

A bar of soap can also become a variant of a Banana Peel when stepped upon.

Examples of Slippery Soap include:

Anime and Manga

On Azumanga Daioh, Osaka washes her hands while wondering if she's really as spacy as everyone thinks she is. The soap leaps away, and a handkerchief she was holding in her mouth falls into the sink.
Love Hina has a Bathtub Scene where Mutsumi is helping Naru scrub and the soap falls in between Naru's legs. Mutsumi tries to grab the soap but ends up rubbing Naru's crotch trying to get it. When leaving the bathtub, Naru slips on a bar of soap on the floor and ends up slamming her face onto the toilet.

Film

In Half Baked, Kenny drops the soap in the prison showers. He doesn't drop it as he reaches for it, but as he is singing into it (yes, this is a strange film).
In The Naked Gun 33 1/3: Frank is in the prison shower, and the Friendly Neighborhood Prison Rapist approaches him and drops the soap. He asks Frank to pick it up. Frank replies "no problem," bending over to reveal Iron Underwear.
Midnight Cowboy opens with Joe dropping a bar of soap while showering.
In Josie and The Pussycats, Melody keeps dropping the soap in the shower. Of course, this has less to do with its slipperiness than it does with Melody's stupidity.

Web Comics

Captain SNES once fell afoul of this...

Web Original

From the SCP Foundation, SCP-1908, aka Mr. Soap. One of the "Little Misters" created by Dr. Wondertainment, SCP-1908 looks human, but he constantly secretes soapy water. Unfortunately, he cannot control this, and often gets it all over the floor, making the floor slippery. When the Foundation located him, he was working in a factory in China, his "talent" making the work environment there unsafe.

Western Animation

Happens to Dilbert. Leads to video of him searching for the sliver of soap in the bath that looks like he is doing something else entirely.

Dilbert: It's so small. I can't even find it!

This happens to Dopey in Snow White and The Seven Dwarfs.
Slipping on soap is one of the lesser indignities Roger Rabbit has to suffer in the opening cartoon in Who Framed Roger Rabbit?.
Peter references this in the first episode of Family Guy when he ends up in prison with Brian.

Brian: How was your shower?
Peter: Oh, I tell you Brian, all of the rumors about dropping the soap are true.
Brian: Really?
Peter: Oh yeah, you can't hold onto that thing to save your life. It was slipping all over the place. Guys were laughing.
Prisoner #1: Hey, there's the guy that couldn't hold onto the soap.
Prisoner #2: Oh, that was classic.

In an episode of The Flintstones, Fred is showering, and calls to Wilma, asking her where the soap is. She tells him it's probably where he left it, on the floor. Too late - by the time she's said that, he's slipped on it. Fred sarcastically responds, "Don't worry, I found it!"

Real Life

Samuel L. Jackson once joked that he worried Mace Windu would die in the third film from this, so he personally demanded for George Lucas to give him a dramatic death. He did.