Possible bug with recurring dates and "starting" syntax
This is what happened to me. I placed a task in with the date "every third mon starting 1/1/2012" and it was set to 1/1/2012. I then clicked the checkbox on that task and it was moved to Feb 19th, 2012 which is a Sunday instead of to Feb 20th, 2012 the correct 3rd Monday.
indeed, there seems to be a bug, I will forward this to the developers. Thank you for reporting it and sorry for any inconvenience it might have caused you.
For now, please try to set up the task with a starting day that's also a monday, for example:
every 3rd monday starting 2/1/2012