This isn't so much a common mistake as a common cause for arguments (as is often the case with words of Latin origin).
The dictionaries treat data as a group noun, meaning information, especially facts or numbers, collected for examination and consideration and used to help decision-making, or meaning information in an electronic form that can be stored and processed by a computer.
Then they go on to confuse matters by giving the following kind of example:-
The data was/were reviewed before publishing.
If you're writing for an academic audience, particularly in the sciences, "data" takes a plural verb.
The data are correct.
But most people treat 'data' as a singular noun, especially when talking about computers etc.
The data is being transferred from my computer to yours.
And I have to be honest, I've never heard anyone ask for a datum.