Hi Jackie! Great question!
Individuals with profound hamstring and/or plantarflexor weakness may have knee hyperextension in mid-stance due to a lack of eccentric control of knee extension (i.e., normally functioning hamstrings or plantarflexors could prevent the knee from snapping into hyperextension).
Of course, knee buckling is also possible with many of these impairments, as knee hyperextension is a compensation to seek passive stability. Notably, foot slapping during loading response is a function of ankle dorsiflexion weakness due to lack of eccentric control into plantarflexion.
Hope this helps!
Chrissy