With VNAV, if the altitude indicator is less than the cruise flight level set in the PERF page (or VNAV page) of the CDU, then the Autothrottle will switch to SPEED HOLD (speed is displayed on the IAS/MACH Indicator) and you will also be in Altitude HOLD mode.

I have had this happen a few times, simply because the Cruise Altitude in the CDU was higher than that which I entered into the Altitude Indicator. I guess it's because I didn't double check everything!
Where was my First Officer when I need him? ;D

Mark