A mail has enough points to be spam, but isn't marked as such!
Typically people who report this are seeing something like the following in the mail headers:
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X-Spam-Status: No, hits=7.0 required=7.0
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It is a little confusing, but mathematically there is no bug here. The confusion is caused by rounding. The message actually scored 6.950-6.999 (scores go to 3 decimal places), but the header only displays to a single decimal place. Therefore 6.9# (# is >= 5) rounds the whole value to 7.0.
If X-Spam-Level is enabled for rewriting, you can see that the integer value of the score is only 6 for this message:
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X-Spam-Level: ******
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since there are only 6 stars.
The rounding issue of the value has been changed for SpamAssassin 3.0; see also RoundingIssuesOrphan. Replaced with RoundingIssues.