MD5 pre-fixing with an unkown postfix
If the attacker does not know the value of $secretkey$, he cannot predict in advance the value of $MD5( attackercontrolled + secretkey)$.  However, what he can to is create two different messages $M$, $M'$ such that:

$MD5( M + secretkey) = MD5( M' + secretkey) $

He cannot predict this common value in advance; however, if he is able to submit $M$ and observe the generated tag, he immediately knows the tag for $M'$

This is a violation of the security properties that message authentication codes are supposed to provide.