well
it happends when i changing some materials at for example : at plane when is displacement of grass but this happends with another situations when displacement is not there… ( only when is chaged material and than hit render)
than my light cache calculation hangs up and calculating and calculating ( like in a never ending story heh) so only way i can stop it is: ctrl alt del and find max.exe and stop the process .
fist bounces i use qmc and sec mentioned light cache with irr map i never find this situation and also with qmc at sec bounce.
ps: sorry for ma english (we learned russian language at base school 
)