In GMail (Google Mail), after composing a message, pressing on the tab button is suppose to take the user to the Send button. This way, Tab followed by Space Bar can send the message. However, in some cases, clicking on the tab button skips the Send button and goes to Save Now instead. At the same time, Shift+Tab key combination sometimes goes to the "To" field rather than the "Subject" field. This error occurs randomly, so try it a few times to generate it.
I tried to reproduce the problem in old UI=1 and in a new UI=2 of Gmail in Webkit r30551. Both keyboard shortcuts work as expected, so I cannot confirm the bug.