I just tried to insert a table while writing a post but it appeared broken in the preview. I used the basic example shown in the "formatting tips" to confirm:
| A header | Another header |
| -------- | -------------- |
| First | row |
| Second | row |
And here's what I see in the preview:
I even tried to edit an existing post that had tables in it and all the tables were broken in the preview (although they looked fine in the live post).
To test this, you can simply edit this question and look at the preview of the following table.
A header | Another header |
---|---|
First | row |
Second | row |