I find the default configurations of most computers to be too low on memory. So one of the first things I did after ordering my PowerBook last year was to order a 1G SDRAM module from NewEgg. (The PB came with 2×256M, and Apple's RAM is way overpriced.)
Unfortunately, last week I started having trouble with it. I had noticed that my laptop was reporting 768M rather than 1.2G as it did when I first installed the module. This was strange; can just half the memory be working? I removed and re-seated the module, with no improvement.
So then I tried swapping my module with the identical one in Art's iBook. His computer reported the full amount (1.2G) and now so did mine. A strange situation, but I thought the problem was solved.
But then the next day my PB experienced a kernel panic. If you've never seen this on Darwin/OS X, the screen dims slightly, and a message comes up in about 5 languages that “you need to restart your computer,” with little additional explanation. Like a stable Linux, this kernel really does not panic lightly, and it is often indicative of a hardware defect.
At first, I'm not sure I blamed the panic on the recently swapped memory module. But then the computer was up just about 10 minutes before it crashed again. The next time, I only got about 8 minutes. This was becoming serious. As a stop-gap, I swapped the 1G module with the original 256M one, and the crashes ceased.
But this is strange: both 1G modules seem to work in Art's iBook, neither works fully in my PB – although they exhibit very different symptoms – but the 256M module works fine. I don't know whether to blame my laptop, the memory, or some crazy conspiracy between them. It's far simpler to order a new 1G module than to phone AppleCare and complain about my laptop's behavior with 3rd-party RAM. For now, I'm just surviving on 2×256M.