I hope i'm using the right terminology here.
Basically from time to time, Windows clients get prompted for username/password when trying to connect to an open share (Debian), running Samba.
The prompt looks like this:
username: 192.168.x.x\Guest (Greyed out)
password:__________
Leaving the pw field blank doesn't work.
However there is no password on any Guest account, it is an open share meaning: annoynmous logon is excepted.
This seems to happen most to Windows clients that are not domain-joined.

Thing is this doesn't happen everytime, only on occasion.
I don't know much about Linux.
Does anybody have any ideas?

I'm not very good with Samba, but you could try setting the security level to 'share' instead of the default 'user'.