As can be seen in the attached image of the Email Voting example from OMG (http://www.omg.org/spec/BPMN/2.0/examples/ZIP/eMail%20Voting/) some of the task boxes do not have complete boundaries, while others do.

Also if you hover over the "Moderate E-mail Discussion" top level task in the first sub-process, the text gets squashed - and opens out again when the cursor is moved outside the task box.