Happens to me too. Every now and then I get an empty message. Note that I say 'empty' and not 'blank'.
Empty as in there is nothiong in it, the message is merely a bunch of headers with the body gone missing. Not blank like in a message with invisible contents.
As I'm using my own mail server, I can check very easily how that message arrived and whether those contents have gone missing somewhere between my server and TB or prior to arrival on my server and everytime I check it's the latter.
Therefore I suggest that it's the internet (for lack of a better word) that ate your message.
Most (99.9999%) of the mutilated messages that arrive on my system are likely to be spam (sent to me from unknown addresses with subjects like porn, drugs and gambling) so I don't feel I've lost a lot with them.
If an empty message would appear be sent to me by a known contact I'd ask him to resend it.
I'm subscribed to several Yahoo groups and in the past it happened that I received messages from yahoogroups with either no body or a empty line somewhere in the headers or without the empty line between headers and body. As RFC2822 defines that the headers end after the first empty line that may result in either a message that appears to be empty or one that starts with some nonsense (part of the headers).
The only way to see what's wrong with a message is by viewing the source. In TB you can do that by pressing F9. If the source tells you that the message should display a message and TB doesn't do that then you've got a TB problem (even though in my experience messages, that only link to an online pic without accompanying text, are always spam) and we can search further, but my best guess is that you don't see anthing, because there's nothing to see.
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I'm just a user of The Bat! I don't work for Ritlabs.