Thank you for your post.
I am unable to duplicate the problem. This is what I have done.
1. I created two tables: Master and Lookup.
2. I related both tables by the text field "ID".
3. I entered many records into Lookup and a few into Master.
4. On my Master layout, I included a portal into Lookup. For each of my records in Master, I have at least one record in Lookup.
5. I turn on Instant Web Publishing and allow access to all users.
6. USER1 launches a browser on a Windows XP machine and accesses the hosted file. USER2 launches a browser on an iMac and accesses the hosted file.
7. USER1 makes changes to the portal records for record 1 (ID=1) as well as some changes to the Master table.
8. USER2 makes changes to the portal records for record 2 (ID=2) as well as some changes to the Master table.
9. USER1 submits the changes. USER2 submits the changes. No error messages. Both records and portals are updated accordingly.
Let me know what I'm doing differently, and I'll try to duplicate the problem here.