I can see how a key might get stuck inside the heyhole. Handcuffs have
a key guidepost within, and the end of the key must slide over the
guidepost before it can be rotated to unlock the cuff. If the key is
then rotated, it has no way of coming out of the lock until it is
rotated back and pulled off of the guide post. As Fred said, it is not
easy to know.
Now, another pet peeve of mine about the depiction of handcuffs on TV
is the length of chain between the cuffs. On TV the length is often a
foot or more, while in reality there are seldom more than 2 links of
chain in between. It often looks like the prop department uses leg
restraints for handcuffs. The long chains are often plot contrivances
that allow for the detainee to step through the cuffs and then choke
their captors. This really pisses me off because no cop or
correctional officer would carry or use something so dangerous.
I can't remember if Kate was bound with similar cuffs on the plane, but
I know Jin was. I am not sure if Jin was cuffed with leg restraints or
just Hollywood handcuffs.