Just asking out of curiosity:
In wp-includes/general-template.php there is the get_calendar() function defined. $group from the cache functions is every time 'calendar', and according to Codex you need it to use the same key (here 'get_calendar') across various groups. Now the key get_calendar is only used in the calendar functions, so isn't the $group variable ('calendar') here actually redundant, since you have the key only in one group? Or is it used for compability with future versions of WordPress?