When you have enough quests in the log that you get a scroll bar. Things get weird now: if you want to link a quest that is outside the main quest window (so if you want to scroll down to link the quest), linking it will completely fail. If you try to link some quests that are still on the bottom visible main scroll area, it will shift itself and link a different quest than the one you want.

This is the quest log with scroll bar:

This is the output of quest shift+click linking in chat. As you see, there is some weird behavior here:

Linking quests outside the main visible area in the quest log will just yield text - no Questie link whatsoever.
This bug doesn't show itself if you don't have enough quests in the log to get the scroll bar.

Questie 7.0.1 - wotlk