As http://spamassassin.org/tag/ explains, sometimes SpamAssassin will get a "false positive", resulting in tagged mails which are not actually spam at all.
Since spammers don't usually highlight the fact that their mail is unwanted, unsolicited ads, SpamAssassin has to try to work it out – and it's not always easy to do. And it's only a computer program, so it does get it wrong sometimes – even humans do.
We use a huge database of mail traffic, and analyse the hit rates of each rule to maintain quality and try to ensure that they only hit spam (as much as possible, at least). Rules that hit non-spam get very low scores.
Tip: examine exactly which rules were triggered. Take careful note of the points listed beside each rule name – low-scoring rules do not make much of a difference; it's the ones with high scores that need to be avoided.
If you are the mail sender, this may help: AvoidingFpsForSenders
If you admin a SpamAssassin installation and suspect something in the configuration may be wrong, see here: AvoidingFpsForAdmins
If you are the recipient, simply add that sender to your ManualWhitelist. (You could also change the score of the rules that were triggered. See WritingRules).