On my site, we’ve been having trouble with the [details] tag where opening it in the preview will cause the block to be open by default.
This is backed up by checking the BBCode for a post, which will have open appended to the tag (as in [details="This should remain closed" open]) if it was open in the preview at the time the post was submitted.
This seems like it defeats the point of the tag, especially since we often use it for spoilers.