Here's an interesting one...
I have a Windows Installer DLL which checks for reboot requirements, thus I'm going to be including it in pretty much all of my installers.
Most CA DLLs that I've written are included through Merge Modules but this one's being included directly in the installer.
The first two installers I put it in worked fine but the third one keeps failing, saying the DLL required doesn't exist.
I've checked the logs and watched the WINNT\Installer directory and it's true. The MSInn.TEMP file is not being created.
I've tried deleting the entry from the Binary table, CustomAction table and InstallExecute sequence and then putting them back in from scratch... still nothing.
Does anyone know of any oddity in the Windows Installer that might do this? I'm going to keep looking for a stupid mistake on my part, but I'm running out of ideas here.