1. Put item frame on block and comparator behind it.
2. Put anything on the item frame so comparator can detect the item frame.
3. Type: /kill @e[type=ItemFrame]
4. Item frame disappeared and comparator doesn't know that item frame is gone!
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Confirmed for 14w34c

Confirmed for pre-3
In 1.8.1 Pre4

Confirmed for 1.8.3
Confirmed.