What happens to 'Crackdown' when it targets 'Traptrix Sera'?
Crackdown has two effects, the first of which reads: 

Activate this card by targeting 1 face-up monster your opponent controls; take control of it, but while you control that monster, it cannot attack or activate its effects.

Sera is unaffected, but only when she's Link Summoned. She can still be targeted. but if she was Link Summoned, Crackdown remains in its activator's field, but it simply does nothing and is destroyed when Sera leaves the field. If she was SPECIAL summoned, however, (like, by card effects), then it's resolved normally.