Definitely, otherwise known as "Definately, defenitely, definetely, difinitely," or my personal favorite, "[i]defiantly.[/i]"
[spoiler]There's not even an 'A' in the word. How can you get that shit wrong?[/spoiler]
OTHERS:
There and Their - Mix-up
Hear and Here - Mix-up
Your and You're - Mix-up
Tomorrow - Tommorow, Tomarrow, Tomorroh (seriously saw this once.)
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pneumonoultramicroscopicsilicovolcanoconiosis.