read-only error

"This document was read-only when opened and may be used by someone else.
If the file is used by another user, registration will fail.
If the file is no longer used and the document is saved under its original name,
changes made by another user are overwritten."

Guys sometimes when i try save it file i see read-only error like that.
I usuing public pc so anyone tryna see it my file or just simple error? SW2016