Maybe I'll save someone some time until there is some sort of integrated fix or if these are just flukes (however, I see them on multiple sites).
On one site with only one smart group: I disabled it I created a new smart group Successfully ran rebuild system cache!
It seemed to clear up an error in our dblog about htmlentities (htmlentities() expects parameter 1 to be string, array given in CRM_Utils_Type::escape() (line 261 of sites\all\modules\civicrm\CRM\Utils\Type.php)) and also mailing issues to that smart group.
Tonight I'll try to disable all of my smart groups on another site, run the rebuild and see if it clears it up there.
Update: On site with multiple smart groups, I went to Manage Groups, clicked on Contacts for each Smart Group and on one group, I got the error message about is not of the type Int.
All other Smart Groups correctly displayed the contacts within that group.
So I disabled that Smart Group and successfully ran Rebuild Smart Group Cache.
If I re-enabled that group and ran the rebuild again, the htmlentities error came back.