Many vSphere administrators have learned the hard way that with default settings, the root account in the vCenter Server Appliance 5.5 expires after 90 days. The VMware KB article 2069041 addresses how to change the root account password after it expired, which requires rebooting the appliance, modifying the grub boot parameters and using the passwd command.
To avoid this issue, you could consider modifying the ESXi host policies, such that it forces the user to change the root account password whenever it expires rather than locking the root account. The KB article 2069041 also discusses how to adjust the expiration policy.
Likewise, some vSphere administrators have unexpectedly experienced situations where the VMware vSphere Single Sign-On (SSO) administrator account is locked due to password expiration. See VMware KB article 2034608 for details on resolving the issue in SSO 5.1 and SSo 5.5.
Naturally, you can avoid certain issues by configuring a solid SSO Password Policy by implementing the procedure found here.