As we all know, the Question mark in IOS is used as interactive help, but what about a ? in a password?
This could for example be a task in your CCIE Lab or at least in on of the many preparation labs.

To use the ? literal just enter crtl+v and then ?

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Rack1SW3(config)#username cisco password SECR?
LINE    <cr>

ctrl+v ?

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Rack1SW3(config)#username cisco password SECR?
Rack1SW3(config)#

Easy stuff but good to know!
Of course the ? can also be used in ACL Names, Route-maps, etc…