1. Britney Spears on ‘How I Met Your Mother’
Before hitting the screen on 'The X Factor', Britney Spears made a quick appearance on 'How I Met Your Mother' back in 2008.
2. Taylor Swift on ‘CSI’
Taylor Swift was almost unrecognizable on 'CSI' back in March of 2009.
3. Miley Cyrus on ‘Two and a Half Men’
Making her first post-hair chop TV appearance, Miley took her acting talents to 'Two and a Half Men' alongside Ashton Kutcher.
4. Jennifer Lopez on ‘How I Met Your Mother’
Jennifer Lopez showed off her comedic side on a hilarious episode of 'How I Met Your Mother'.
5. Demi Lovato on ‘Grey’s Anatomy’
Demi Lovato played a troubled teen in her 2010 guest stint on 'Grey's Anatomy'.
6. Chris Brown on ‘The OC’
Aw! How cute was little Chris Brown on 'The OC' back in 2006? The good ol' days.
7. Carrie Underwood on ‘How I Met Your Mother’
Carrie Underwood also appeared on 'How I Met Your Mother,' playing a super hot stewardess.
8. Joe Jonas on ‘Hot in Cleveland’
1/3 of the Jonas Brothers, Joe Jonas, did a two episode stint on the hilarious TV Land show, 'Hot In Cleveland'. We don't know who was cuter on the show; Joe or Betty White?
9. Adam Lambert on ‘Pretty Little Liars’
Adding to the creepiness of "The A Train," Adam Lambert tossed on a pair of fangs and performed on the 'Pretty Little Liars' Halloween special.
10. Justin Bieber on ‘CSI’
Justin Bieber wowed us with his dramatic 2011 guest role on 'CSI'.
11. Jennifer Hudson on ‘Smash’
What better show than 'Smash' for 'American Idol alum, Jennifer Hudson, to have a guest role on?
12. Katy Perry on ‘Raising Hope’
Katy Perry had us giggling all night with her hilarious appearance on 'Raising Hope'. Fun fact—Katy's BFF is the show's star, Shannon Woodward!