[jcassity]

the only problem with that is ohms law.
two resistors in parallel means,,,

if you have a need for 500k resistor and your color bands say green, brown yellow, but the meter says you are at 310k,, the resistor is drifting towards a short.
if you insert a parallel 500k resistor,, ohms law says the the total resistance of 2 or more resistors will be equal to or less than the smallest resistor. inserting a parallel 500k resistor would not fix the issue, it could actually cause the total resistance to be less than 310k!

snip the old resistor out almost flush the the resistor itself, pig tail the old resistor ends, insert your new resistor into the pig tails,, solder up and trim.

if your resistor is spec'd to be 500k and you read 700k,, then yes a parallel resistor of another custom value would be a way to calibrate that particular portion of the circuit. by keeping the out of spec 700k resistor "drifting towards an open state", you would reasoable parallel in a 1.7Mg Ohm resistor to calibrate it down to 500k.