I've posted bug report to Postgres mailing list (BUG #5289: Unpredictable error in plpgsql function loop).
Then I found even more simple code to expose error, so I've reported next bug (#5290, now waiting for approval).
What can I expect? When there be any response to this report? And if it will be confirmed - when to expect fix for it?
Put it in other words: how responsive is Postgres support / how long it takes for them to fix a bug?