I'm responsible for maintaining an extension to ArcMap that consists of a toolbar with a bunch of menus & commands & tools. It uses the old-style COM registration for all of its UI. Recent installations on our test machines have intermittently refused to put the toolbar in the list of toolbars that users can select. It is listed for me (a local admin) always; for others sometimes, and for still others never.
Even when the toolbar is not listed, most of the custom commands and such (likely all of them, but there are several dozen and I'm too lazy to take a definitive tally) are available in ArcMap's Customize window.
At first I assumed that the COM registration was failing specifically for the toolbar, but there is a registry entry for
HKEY_CLASSES_ROOT\CLSID\MyToolbars.ProgID.MatchesMyToolbars.NetNamespace so I think everything looks okay there.
Where else might I look to find a cause of this issue?
Updates & Fresh Breadcrumbs
Based on Kirk's advice in the comments I confirmed that the toolbar is registered using the categories.exe utility.
A coworker discovered that if he enables the extension that comes in the same library as the toolbar, closes ArcMap, then reopens ArcMap, the toolbar appears in the list. Deleting the user's profile indeed resets the problem.
A well-meaning sysadmin found that running CCleaner clears up the problem, but they ran it wholesale and made no log of what was changed. On the one remaining test machine that wasn't CCleaned, I re-did that cleaning incrementally to determine which piece of flotsam causes the problem but came up empty-handed.
All but one of the test machines (the one that I cleaned manually in (3)) have been rebuilt from a new image and the problem persists. If anything its intermittence has decreased, and the toolbar is more likely not to be available.
After the rebuild I was able to determine with a moderate level of confidence (still some intermittence) that the installation protocol I use on our test computers contains bugs. Working on a potential fix now.