This is a known bug [Bug 258465]. What’s worse is there seems to be no pattern. At least something to help coders identify the source of the problem, so they can figure out how to fix it.
If any Mozilla Thunderbird users out there have any info on what the cause may be, please provide info on the bug 258465 page. (Please only comment on that page, if you have helpful info. No “me too”, or “I hate this bug” posts.)
Bug 258465 is marked as a Thunderbird 1.1 blocker. Thunderbird 1.1 isn’t planned to be released for a long time, but the fact that is flagged as a blocker should be an indication of priority.