master-slave flip-flops
A latch may guarantee that if the clock input has a rising edge and remains high for at least Twidth, and the data input is high at least Tsetup before a rising clock edge and remains high for at least Thold after, then within Tpd the output of the latch will be high and remain so until the next rising clock edge.  That does not mean, however, that the output of the latch will never change at a time more than Tpd removed from a rising clock edge.

In an ideal world, if a latching circuit receives a stimulus which may or may not cause it to change state, one of two things would always happen:


The stimulus will visibly change the latch state immediately; the latch will remain in that particular changed state until the next stimulus.
The stimulus will not change the latch state at all; the latch state will remain unchanged until the next stimulus. 


Unfortunately, there's another possibility:


The latch state may be changed in a way that is not by immediately or fully apparent for some time; the visible state may change at some arbitrary future time, unless a "clean" stimulus puts the latch into a known state.


To use a crude physical analogy, if a standing bowling pin is hit soundly, it will fall over immediately.  If it's hit very weakly, it may be nudged slightly off vertical, but will quickly return to standing firmly.  If it's hit somewhere in the middle, though, it may wobble around awhile before either returning to a steady upright state or toppling over.  It may not be apparent for a over a second what the pin would end up doing if left undisturbed.  The worst-case time to determine if a pin has been hit hard enough to topple it may far exceed the time that would be required to determine a clean hit or miss.

The closer a latch's stimulus is to one which would either almost or barely cause it to permanently change state, the longer it may take the latch to produce a cleanly-resolved output.  

It would seem like it would be possible to design a latching circuit with a three-phase output, where the three phases were "high", "low", and "wobbly", and have downstream circuitry wait for the latch to produce a clean output before using the output value.  Unfortunately--returning to the bowling analogy--while it may be possible to tell that a pin has fallen over sufficiently that it's not going to right itself, and it may be possible to tell that a pin has sufficiently stabilized that it's not going to fall unless it's hit again, there no clear and unambiguous way of establishing the moment where the pin's outcome becomes certain.  No matter what one does, there will always be a theoretically-possible "trouble case".  The best one can do, oftentimes, is let the pin sweeper come in and reset the state of the system, and hope there's no disagreement about whether the pin should be counted as having fallen before the pinsweeper got it.