Moved out of 0.8 congratulations thread into its own thread. Please open new threads for new problems
I've sometimes seen similar behaviour with my ePub firmware Cybook. I was (mostly) able to fix this by either changing conversion options and reconverting or fixing some offending HTML inside the ePub.
What this boils down to is that it is most probably due to a faulty conversion source and almost certainly not due to a Calibre bug, though to be sure, you'd have to look at the MOBI file(s). If you're converting, what format are you converting from?
You could try asking in the MOBI forum section