thanks guys for your thoughts. this is primarily for my own use and my own projects; although occasionally, i'll use my own stash for a client's project (i'm a consultant) if needed and bill them for it. of course, I'm buying used RAM off eBay (mostly) and one of the main goals is to validate that the RAM is good, so I can return it within the return window if it is not good. secondary goal to that, is to reduce the chances of having RAM related problems when my servers are deployed for a project. None of my projects involve life threatening situations or weapons systems these days; so nothing terrible will happen if a system fails, it would just be an inconvenience, perhaps delay in a project, and some annoyance from a client.
i've been thinking about all your comments and my original question, and I think I probably didn't really make it clear and I've been thinking about it some more. so, here's my re-phrasing of the question:
"if you regularly test RAM with memtest86, how often have you encountered RAM modules defects that showed 0 errors during the first half (50%) of the tests, but started showing errors in the 2nd half of the tests? how often have you encountered defective RAM modules that were detected in the first half of the test?"
To normalize your response, can you estimate an order of magnitude of how many DIMMs you've tested during the period where you've encountered the above error condition(s). For example, for me, I've probably tested about 100+ DIMMs in the last 12 months, and in that period, I've encountered I think 4 defective DIMMs (definitely less than 10), but all 4 were detected during POST (before running memtest86), none were detected by memtest86 during first or second half of the test.