If you try to delete chords with DEL key ("note entry mode" and "normal mode") you cannot use the left and right arrow keys afterwards. You either have to reclick a note (normal mode) or exit "note entry mode" + reenter with N.

Here is a longer description:

Precondition:
Chord with at least two notes in one staff.
One note to the left, one note to the right.
Not in note entry mode. "Focus" on chord.
→ it is possible to move with left an right arrow keys.
Delete one note within the chord with Del key.
→ not possible to move left and right with arrow keys (select of notes lost), you have to select a note once again to be able to move left and right.

Solution (in my opinion): Delete whole chord with DEL-Key or move Focus to a different note within the cord.

Note entry mode:
Chord with at least two notes
chord is hilighted (in Focus).
press DEL key.
One note of the chord is deleted.
Error:
Arrow left/right do not work anymore.

The same happens for me, if you try to delete a rest with DEL in entry mode.

I use Ubuntu with musescore 1.0.