I have experienced something like this on our SMS 4200 appliances. I am upgrading them from 7.5.0-26 to 7.6.0-6 as I write this, but I've experienced this since at least the first 5.0 release.
For me, it doesn't happen all of the time. In fact, I can't say that I can replicate it reliably.
We use an administrator only quarantine, so once or twice a day, I'd go through the quarantine releasing or deleting messages. I'd open the top message in the quarantine and click the delete or release button as appropriate and the next message would appear. I began noticing that I would see duplicate message bodies from one message to the next, which is not unusual, but then I noticed that occasionally, the subject would be different. I had been deleteing legitimate messages that I thought were SPAM and releasing SPAM that displayed the body of a previous good message.
I've reported it to Symantec, but have never heard back and it has never been fixed. I even recorded a screen capture video of it happening and sent it to them. It does appear to happen somewhat less these days, but I still have to pay close attention as it could pop up at any time.
The ONLY way I could force it to show the correct message body is to go back to the quarantine list and come back into the specific message.
You didn't mention the version of SMS you were using so I don't know if this will help. (Does anyone else wish that Symantec would split the forum up into Appliances and Software? A lot of information doesn't seem to apply between them.)