When a loaded package has a DLL, install() will attempt to overwrite the DLL. This works fine on Unixlike systems, but it fails on Windows, with an error that isn't obvious. In these cases, devtools should try to unload the DLL and/or give a clear warning message.
This problem is unavoidable when a package which devtools depends on (e.g., plyr) has a DLL. It's also unavoidable with current versions of devtools, since devtools itself has a DLL now.
Is this supposed to be filed under plyr?