1) When your girlfriend or mother makes you carry all of her shopping. Even if it's only three ounces, it's way too heavy.
2) When the events of the day make you feel suicidal, but you're too wiped out to put yourself out of your misery.
3) When your RPG character can no longer move due to having an absurd amount of stuff in his tiny backpack.
Note: Some RPGs are gracious enough to allow you to move slowly while over encumbered, but it's so slow you might as well be standing still.
"Crap, man, I'm over encumbered."
"Well, leave the iron hammer."
"No way! That item is vital to a later side-quest. I'm not coming all the way back to these caves to get it."
"Well, leave the shield. You can pick up another at Cloud Ruler Temple."
"Nah, I'll need that for getting out of this cave alive."
"Drop the gem then. It'll be just enough to put you under the weight limit."
"Man, this thing's worth 2,300 gold! I'm not just gonna drop it."
"Then quit playing Oblivion and go outside and get some fresh air."