a) A user can get the "URL is Old" error message in Matrix if they have been inactive for an extended period and then try to start working again.
b) It can occur if there is an interruption in your internet connectivity while in Matrix. If you lose your connection, even for a second or two, it can confuse Matrix and generate this error.
c) It can occur if a user starts doing an input/edit and also start using Matrix in a separate tab. Recommendation is to complete inputting or editing prior to opening/working on another matrix tab.