If you moved to Mac OS X from just about any other operating system, you are probably used to the trash can or recycle bin staying put when you try to put something in it. On Mac OS X, however, what would normally be a half second trip to the trash can become a ten minute ordeal as you try to remove the 50 tiny icons you've just accidentally placed in your dock.
If you don't know what I'm talking about, then good for you, you are paying a whole lot more attention to where you are throwing your mouse than I do. Since the trash can in Mac OS X is in the portion of the dock which allows you to place documents in it, if you try to throw away a bunch of documents and miss the trash can by a couple of pixels then instead of ending up in the trash, they end up in the dock.
Innocent and unsuspecting dock.
Trash can being narrowly missed.
New Addition to the dock (luckily I was just trashing one thing).
And to the rescue, comes the command key. For those of you unfamiliar with the Mac OS Command Key, take a gander at the key with the apple and/or the little doodle on it. Among the myriad uses that this key has (menu shortcuts being just the tip of the iceberg) is the ability to make the dock behave itself.
While holding down the command key, drag your trash to the trash can and nothing will move around. Try the command key out in other situations, you might be surprised at what it can do!