This is the first error I encountered while upgrading vBulletin to 4.2.0.
Due to the following errors, the install/upgrade can not continue
Following is a list of files that are either missing or contain unexpected contents. The most likely cause is that all of the files from this release have not been properly uploaded. You should resolve this error before continuing.
<list of bunch of files in /clientscript/.. and other folders
I ignored it, as suggested by others who encountered this error, and went ahead with upgrade.