When you delete the first/last row in acp using ajax, the moving options of the new first/last are not disabled.

Example:
Item1 - No-Up - Down
Item2 - Up - No-Down

If you delete Item1, you get:
Item2 - Up - No-Down
but you should get:
Item2 - No-Up - No-Down

If you delete Item2, you get:
Item1 - No-Up - Down
but you should get:
Item1 - No-Up - No-Down