RETR error: Can't open the message file - it's gone!

[#1]

Under what circumstances would the plus.net mail server respond "Can't open the message file - it's gone!".
Have I lost an email?

[#2]

I got that message a week ago but I don't think I lost any mail.
When I checked my mail logs I found that the error occurred when Thunderbird tried to download message number 23 out of the 27 new messages in my inbox. Thunderbird gave up at that point so I could only see 22 new messages in Thunderbird.
A few minutes later I tried checking for new mail again and this time Thunderbird was able to retrieve the missing 5 messages (if I remember correctly the log showed 27 messages waiting and Thunderbird retrieved numbers 23 to 27 this time).