Yes, when adding.
To clarify a bit:
When you add a feed to ThunderBird it will attempt to validate it first. If it fails the validation, it simply will not let you add that feed. Even if you know that it's a feed that normally does validate, but doesn't validate at that particular instance in time for whatever reason (site down or whatever). This has been a long-standing annoyance with ThunderBird's handling of adding feeds.
You might wonder what happens if a feed that has already been added, suddenly no longer validates - it will throw an error in the status bar about it, and ignore the feed. Once it validates again, feed entries are once again parsed.
This can lead to a situation where e.g. there are 4 new posts, and the latest post causes the feed not to validate, all 4 of those posts being 'lost'
and however many posts are required before that particular post is dropped off the end of the feed having to be listed before it parses the feed entries again.
Technically, this is an SMF issue. Practically, this is a ThunderBird issue. Realistically, this is an end-user issue as neither of the aforementioned are likely to be addressed any time soon..use a different feed reader
(or an older version of ThunderBird, I guess).
Example, the Hebrew forum section right now:
https://bitcointalk.org/index.php?board=95.0;type=rss;action=.xml;sa=news&limit=100Error: not well-formed
Source File: https://bitcointalk.org/index.php?board=95.0;type=rss;action=.xml;sa=news&limit=100
Line: 127, Column: 278
Source Code:
<![CDATA[פורומים ואתרי מסחר ישראלים בדרק נט עם תשלו]]>
It specifically complains about the ending of that - "" .. it's expecting that to end with a semicolon, as a numeric character reference. The forum chops it by output character, rather than input character, and so the feed fails.
