Some history here:
I am updating the metadata for a large number of books manually, and I had some issues entering the ISBN for 3 books, which I had the ISBN's for but the automatic download metadata button could not find. I decided to enter the ISBN's manually into the ID's field. When I did so, the ISBN's would not save. The ID's field did not turn red like it said in the manual if there was an error, and there was no indication of any errors, when I pressed OK to save the change. It turns out that I was not entering the prefex "ISBN:" to the number when I was entering it.
I would like to propose a suggestion for the updating of the Calibre user manual in regards to the ID metadata field for user clarity.
Currently the manual reads under
Editing the metadata of one book at a time:
"The ISBN box will have a red background if you enter an invalid ISBN. It will be green for valid ISBNs"
I propose that the manual change to read something like this (wordsmith away, I'm by no means the best when writing clear concise instructions

):
The ID's box, used to enter ISBN's, and other identification codes will have a read background of you enter and invalid value. It will be green for a valid value. The books identifier must have a valid prefix to identify the type of identifier being entered. For example the ISBN for the book IT by Stephen King would have the id of "ISBN:0451169514" in the ID field.
Valid prefixes followed by ":" are:ISBN - international standard book number
ASIN - amazon identifier
google - google identifier
(etc. - an updated list of what's currently valid or just the most easily entered which I don't know)
I feel with this clarification, for those that actually read the manual, that might make it easier to figure out how to get a correct number entered manually. I only noticed that I didn't enter the prefix (after checking to see if I could see anything in the manual about the ID field metadata field), when I tried a copy/paste of the ISBN number using the paste contents button, and then realised (I was looking at them all along...
*silly me*) that I had not entered the prefix.