Thomas and Boris thanks for your comments.
Here are a few more details.
It is very hard to test, because it does not happen for me.
I have even trotted my laptop around to various cafes /
wireless environments trying to provoke the problem,
but I have not yet experienced the problem.
It is not SOP, because the web page and the web server
are coming from the same domain + server.
It is (seemingly!) random. I have talked to two users to verify this.
When an RPC call fails (e.g writing a record), if they try the same
call a few seconds later, it might succeed.
One of the apps subject to this problem has been in production
since 2008. This problem has appeared only recently. I think the
first time a user wrote in about was in Dec 2011, and since I could not
replicate what he was talking about, I attempted to dismiss it as
related to his strange bad internet connection. Since then many
users have reported it. There have been certain days when it
seem to happen a lot.
I have some tried some weird stuff, like iterating on the RPC
call while it is failing (try - fail - wait - try again ... 4 times).
I can't tell if this ever does any good, but from what the user
told me yesterday, sometimes it goes through 4 tries