I can confirm this in Windows XP.
"Action: Insert file into a container field, storing only a reference to the file."
This action puts two paths into the container, first one being something like: File:FileName.txt
If a file named FileName.txt exists in the same directory, FileMaker will open it.
"I specify a full path for a file to be inserted into a container field."
If you put ONLY the second path ( something like: filewin:/C:/Documents and Settings/Raybaudi/Desktop/FileName.txt ), I think that you'll solve the problem.
Interesting, but I'd still call this one a bug as using Insert File from the insert menu doesn't give you any simple way to omit the relative path reference. Frankly, including that reference automatically makes no sense here.