The fighters who have missed weight the most times in the UFC (modern era - post 2004):
This is updated to be current (Feb 26, 2018).
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5 times:
Charles Oliviera - UFC FN50, UFC 152, TUF 20, UFC on Fox 17, UFC FN98,
John Lineker - UFC 163, UFC on Fox 3, UFC FN30, UFC 183, UFC FN96...