If an enabled extension is disabled it can cause the ACP (and probably the MCP, UCP too) to fatal error.

This happens if the disabled extension has ACP modules, and the module is the first-top level module on the ACP page.

Example, an extension has its own ACP module to ACP_CAT_DOT_MODS. It is the first or top level module assigned to ACP_CAT_DOT_MODS, so when that ACP tab is loaded, the page fails. The module is still loaded, but the data it is looking for is no longer accessible, so you get errors notices about undefined indexes, etc.

Likewise, even if your Module is not at the top of a module management tree, a disabled extension leaves behind raw lang vars of its module names in the ACP, and if you click on one of those links to that module, you get the broken module page.

When an extension is disabled, you should not see any artifacts of its existence in the board - including its ACP/MCP/UCP modules.