What version of Moodle are you trying to install or upgrade to?
Probably you are upgrading to Moodle 2.0, but you still have the HotPot module from Moodle 1.9. In which case, you need to download and install the HotPot module for Moodle 2.0:
Hotpot (plugin)
Error installing/updating to Moodle
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