Most common? Accounts.
Most folk forget to create the account on both machines to setup that share. The password is the account's password.
Let's say your are "datche". You must be able to log on to each machine as "datche". That's step one. Now to share something, you log onto the machine you want to share from as "datch" and create the share. Now logout and go to the second machine, logon there as "datche" and connect to the share.
Most folk I know forget the account setup.