I'd been granted full control of a department site created by someone else and had been attempting to clean up permissions by removing unnecessary or repetitive user groups. I'd created separate user groups for an ownership group (consisting of myself and a couple others with full control) and a group with read rights (so, default users, everyone else within the company); I then removed everyone else since the other groups were unnecessary. This did include both the Records Center Web Service Submitters group and System Account (SHAREPOINT\system) user, since I was not under the impression removing either would affect subsequent access for anyone else, and certainly not myself.
Immediately after doing so, I tried accessing the home page for this site (both via direct link, and the "long way" through my department's site, and received the error message stating I do not have access to this site. Confused, I still had the direct link handy for the permissions page...and was able to bring that up successfully and edit permissions as normal (including adding back all the redundant user groups I'd previously removed, or adding myself outside of a group with full control). From the Permissions page, I can also directly access other parts of the site (not just the All Site Content page, but other things within the site, like lists, libraries, etc.). Despite all that, I am still blocked from accessing the home page. For some reason, I cannot access a "Templates" link from the Quick Launch, but can access it from All Site Content.
Did I do something wrong when I removed all those other user groups, specifically the Records Center group and System Account user, or would that have nothing to do with it? Is there something else I'm not thinking of?