As a controller it is shocking seeing how often VFR aircraft primaries (radar return) pass each other, often at the same altitude indicated on their mode C. I can't count how many times I've issued traffic to VFR pilots receiving radar advisories only to have to issue a traffic alert because neither pilot saw the each other.
As for M14P's suggestion to fly 100 ft higher or lower there is an danger in that idea too. An aircraft's mode C can be off by 300 ft and still be considered within tolerance.
Now here you are flying along enjoying yourself, at what you think is 4600 feet, but your altimeter is off by 200 feet so you are really at 4800 feet. The other acft is supposed to be at 5000 feet (assume he is IFR) but his altimeter is off by -200 feet so he is really at 4800 feet too. Radar issues traffic to you, "traffic 12 o clock 5 miles C172 at 5000" the other pilot is issued "traffic 12 o clock 5 miles PA28 at 4500", both of you think hey, we're ok, we are seperated by at least 500 feet and it's a big sky out here, so neither of you bother to look for each other.
You both merge at 4800 ft, and hit.
I had a private pilot visiting me in the radar room one day who use to do the 100 or 200 feet higher thing. As he was sitting next to me as I was working radar he listened as I issued traffic to two airplanes, as their targets merged one of the pilots said, "holy !!!!, that was closer than 500 feet. I said your mode C shows you at 6000 and the other acft at 5500. The other pilot said well I usually set my altimeter so that I fly alittle higher than normal so that I don't run into someone else.
Mike
NATCA FWA