Same note in different voices and lengths plays only the length of the shortest note
If two notes (first and second voice) share the same pitch and the longer duration is the second voice, the second voice will end when the first voice note ends. See the attached score as an example. This issue was brought forward in this posting: http://musescore.org/en/node/12965
I've personally had this happen but could never find the cause until Joe's sample. This bug affects both branch and trunk.