I think I've discovered something broken with CiviVolunteer's views integration, which was only laid bare in the past two civi 4.6 releases (.17 and .18, related to the custom group work).
The symptom is that the views interface now returns "The website encountered an unexpected error. Please try again later." and in the watchdog, I can see this: "CRM_Core_Exception: Invalid Entity Filter for Activity in CRM_Core_BAO_CustomGroup::validateSubTypeByEntity() (line 645 of [...]CRM/Core/BAO/CustomGroup.php)."
Actually, I updated the exception code to include 'Activity'. That part of the code shouldn't be executing except for contact entity types.
Tracing back to how this might have happened - it appears that civivolunteer creates the custom field group that this is breaking on, and it includes a number in the "extends_entity_column_value" of civicrm_custom_group, which I suspect is why this code is executing.
I haven't tried removing this column data, in case it's used in a way I don't understand, but I'm guessing this is the easy solution.
Anyone else have this experience and/or can give me an opinion on my diagnosis?