changing note value in voice 2 causes whole rest to appear in following measure
1. Fill two measures with notes in voice 1 (note value doesn't matter).
2. In measure 1, voice 2, enter three quarter notes.
3. Press Esc to exit note entry mode.
4. Change the third quarter note in voice 2 into a half note.
Result: A whole rest appears in measure 2, voice 2.
Tested using MuseScore nightly revision 5b6da6a on Kubuntu 14.04 64-bit.