Long ago, I was typing away on my Apple //e and Applewrite, and I noticed that one of the words was misspelled – instead of garden it said farden. So I corrected it and went on. Then later, I noticed it was still misspelled! So I corrected it, and went on. Just out of curiosity, I went back up and checked it later, and it was misspelled!! I said WTF’s happening here! This ain’t right! Well, it turned out that there was a bad bit in a memory chip. Once I found that, I swapped it out and things were back to normal.
That was before the day of error detecting and correcting memory. But that kind of error could really ruin something! My confidence was a bit shaken by the fact that something like that could happen. But the old Apples didn’t have any memory tests during power on. Nowadays everything has. So if you have one of those old computers that has no power on self test, you should periodically run a memory test program overnight that thoroughly tests the memory.