Deleting an instrument in "Instruments in Parts" ("Parts" pop-up) may cause the modified part to display too many staves.
Steps to reproduce:
1) Create a score with multiple instruments, e.g., Vocal, Piano, Trumpet, Sax
2) Open "Parts" pop-up
3) Click "All Parts"
4) Try to change the Piano part to contain the vocal on top, e.g.
a) Click the piano part in "Select Part"
b) Add the Voice by clicking "Voice" in "Instruments in Score", then the "+" button
c) In "Instruments in Part" box, remove the "Piano" part by clicking it, then the "-" button.
d) Add the Piano back (at the bottom instead of at the top) by clicking "Piano" in "Instruments in Score", then the "+" button
5) Click the "OK" button to accept and close the "Parts" pop-up.
6) Click on the "Piano" part to view that part. The "deleted" piano part is there (with the staves reversed - bass above treble), but the displayed data is from the voice and piano as selected above.
7) Reopening the "Parts" pop-up shows the piano part twice.
The piano part in the attached file was saved after step 6.
This has only happened once, but I've not tried to reproduce it.
I can work around it by deleting the part and recreating it with "Create Single Part"