combinational circuit interference
The behavior depends on the delay time through all of the various paths from input to output. If there are some short delay paths and some long delay paths then the input must remain valid for the entire time that the combinational circuit is settling. However, if all of the delay paths have the same length then you can change the input value before the previous output has become valid. This is called wave pipelining. It's pretty difficult to do in practice, so we usually add registers in between the input and output in order to accomplish pipelining.