Missing Main Character

The inverse of Can't Drop the Hero -- rather than the game forcing you to use Bob at all times, the game makes you pick someone other than Bob to be the hero for one reason or another. Popular storyline reasons include Bob going missing, being incapacitated, or just not being allowed in whatever town the rest of the group is going to.

Forcing Bob out of the party can lead to serious issues if your team always consisted of Alice, Bob, and Carol and you never leveled up anyone else-- Now Dave, who is at level five, has to try to fight level fifty monsters ...poorly.

See also And Now for Someone Completely Different if Bob's time as the main character simply ends because his story is over.

Examples of Missing Main Character include:

Some parts of Sonic Chronicles force you to use a party that doesn't have Sonic in it.
The fangame Crimson Echoes forces you to fight solo with Marle for a single chapter. The developers admitted that Marle was not intended to be a solo character.
Towards the end of the main quest in Chrono Trigger, Crono is removed from the party, forcing the player to choose another party member to replace him.. Fortunately, due to Leaked Experience, the character will be the same level, but they won't have as many abilities as the characters that the player was using on a regular basis. Once the player reaches the point that only Sidequests and the final boss remain, it is possible for Crono to rejoin the party.
In Final Fantasy VII, there's a short period of time where Cloud is removed from the party and presumed dead, and later found in a coma. During these times, Tifa and Cid take turns being the party leader.
Knights of the Old Republic II did this when you had to attack two installations simultaneously: one on the planet surface and the other, on the planet's moon. Basically, you had to decide which three of your teammates would go to the moon and which two you take with you to the planet. The choice was made slightly easier by the fact that KOTOR gives equal amounts of XP all characters, even on the Ebon Hawk but there was still the small matter of all the cool items being concentrated in the hands of your Top Three characters...
There's a section in Baten Kaitos: Eternal Wings and the Lost Ocean where the player has to take control of Xelha while Kalas, the main character for the other 95% of the game, does various spoiler-y things.
While Dragon Age Origins normally has Can't Drop the Hero in effect, there are a few portions of the game in which the Warden is removed from your party. The first involves entering the Fade to kill a demon -since only Mages can enter the Fade voluntarily, if your Warden is a Rogue or Warrior you have to send someone else. The second is a mission in which your other characters have to break the Warden out of prison (though you can also choose to escape as the Warden). The third is a Duel Boss Combat by Champion against Loghain, which can be fought as any party member other than Dog. And finally, a small segment of the final battle has whatever party members you didn't take with you Hold the Line against the Darkspawn to buy your main party time to kill the Archdemon.