Respell pitches mode adds note to chord when input is by mouse.
It seems that if you enter new notes with the mouse in Replace Pitch mode it adds the note to the chord instead of replacing it.
See this reported issue in Support and Bugs.....
Steps to reproduce.....
1. Start a new score
2. Enter some notes in one of the staves
3. Return the cursor to the beginning
4. Enter Replace Pitch mode (pressing N first if you left note entry mode)
5. Enter some different pitches with the mouse.
Expected result: The notes are replaced.
Actual result: The notes are added to the chord.
Windows 8.1 Pro
MuseScore 2.0 Final