Failures and mistakes, duty and responsibility, losing an advantage, ego-related conflict, miscellaneous challenges 

There are different kinds of bets that fall into a wide range of possibilities, from harmless and trivial (running around the house in your underwear) to those with long-reaching consequences (losing a large amount of money, someone's reputation being destroyed, etc.). When using this conflict scenario, carefully consider your characters, the stakes, and what you need to accomplish in the story or scene.

Being teased by others
Never being able to live down what happened
Becoming the butt end of a joke
Losing esteem in the eyes of others
Missing out on an opportunity
Having to change one's plans
Having to ask for help to fix things, which increases the embarrassment
Embarrassment from the consequence of losing the bet (having to sing in public, wear a humiliating costume, shave themselves bald, wear a rival's sports jersey, etc.)
Having to give a prized possession to the winner (a car, a special backstage pass, a reservation that was impossible to secure, a specialized piece of equipment or type of technology, etc.)
The character publicly having to admit they were wrong
Being forced to support a competitor's goals, ambitions, or ideas
Having to cater to or serve the victor
Having to take on an extra burden or responsibility that requires time or money
Agreeing to step aside or concede in a competition, a relationship, etc.