It is not uncommon for MemTest86 (or any software based RAM tester) to find no errors but the RAM is faulty, or does not play well with other sticks.
But I am not aware of any instance where errors were reported (even if just 1 error) and the RAM will still good. That is to say, if any error, even if just 1 error is reported, the RAM is bad.
I agree with the others and you need to test with 1 stick at a time and hopefully, through the process of elimination, can locate the bad stick.
This can be a tedious process so don't let your guard down. Be sure to,
each and every time, unplug the computer from the wall and touch bare metal of the case interior BEFORE reaching in to remove or insert the RAM, and BEFORE picking up the RAM sticks. And be sure to always handle the sticks at the edges - do not touch the electrical contacts.
Note to
conclusively test your RAM, you need to use sophisticated and
very expensive test equipment, like this
$2,495 Memory Tester (and that's for the cheap model)! So it is usually easier (and cheaper!) to swap in known good RAM and see what happens.