PKSFX: (W18) Warning! OEMNADAP.INF already exists. Overwrite (y/n)?

The least you need to know: if you receive the above error when extracting Windows NT 3.51 Service Pack 5, use the /b flag while extracting.

More detail

For those of you running Windows NT 3.51 (the last version of Windows that separated the kernel and UI subsystems), running the self-extracting SP5_351I.EXE file in a temporary directory from the likes of File Manager will result in the following error:

PKSFX: (W18) Warning! OEMNADAP.INF already exists. Overwrite (y/n)?

Typing Y here will extract the rest of the files, though when you run UPDATE.EXE you'll be given the following error.

An error has occurred: Unable to open the file OEMNADAP.INF

The solution is to use the /b flag, which extracts files into a tree of directories, rather than just putting them all into one place:

SP5_351I.EXE /b

Why Microsoft decided to use software that doesn't extract in this fashion by default for archives that clearly have files with the same name in different places is nothing short of baffling.

Microsoft identified this bug in their Knowledge Base article 149306.