If the day-of-week in a CronExpression contains a week number, the week number causes all but the first DOW to be ignored, and applies itself to that first DOW regardless of which DOW it was specified for. You cannot specify a CronExpression with a DOW for multiple weeks of the month (e.g., first and third Sunday of the month); you cannot specify a CronExpression with multiple DOWs if you have a week number. The following expressions appear valid according to the docs, but do not work:

0 0 8 ? * 1#1,1#3
Should be first and third Sunday of the month
Results in only third Sunday of the month

0 0 8 ? * MON,WED,FRI#3
Should be every Monday and Wednesday, plus third Friday of the month
Results in third Monday of the month, no Wednesdays or Fridays at all

0 0 8 ? * MON#1,THU#2
Should be first Monday of the month, second Thursday
Results in second Monday of the month

These results were obtained by examining the Date objects returned by TriggerUtils.computeFireTimes()

I've also reported this on the forum:
http://forums.opensymphony.com/thread.jspa?threadID=360264