For those who are new to the FNAF fandom, in FNAF 2 you can wear a Freddy mask to get rid almost any of the animatronics, and the one Scott (the creator of FNAF) thought should be immune to the Freddy mask...was Withered Foxy*.
Not Toy Freddy, the one that's supposed to be more advanced
Not Golden Freddy, who's supposed to be one of the most powerful characters in the lore
Not even Withered Freddy, even though it's his own head
Withered Foxy is is the only one** immune to the Freddy Mask
*In case you new fans were wondering, Mangle is Toy Foxy
**the Puppet/Marionette is also immune to the mask, but if she's anywhere near your office, you're pretty much dead
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