The buttons (and the container itself) of the first example are not in the correct place:
https://en.wikipedia.org/wiki/WP:Sandbox?oldid=631736318&action=edit&preview=yes
The second one, which is not inside a table, is ok.
In the broken example, the container seems to have some border spacing.
I see the same problem on ptwiki, using Firefox 33 or Chrome 38:
https://pt.wikipedia.org/w/index.php?oldid=40416350
Version: unspecified
Severity: normal
URL: https://en.wikipedia.org/wiki/WP:Sandbox?oldid=631736318&action=edit&preview=yes