Obli-negated
When you are released from a task or obligation, one which you volunteered happily to do because the other person needed you, but by circumstance and powers not in your control you do not have to do it, but still get credit for being ready, willing and able.
My plans got obli-negated when I promised my neighbor I would walk her dog while she was away. Her trip got cancelled so I didn’t have to do it, but now she thinks I’m a good person.
A prefix which should, but does not, negate the word to which it is attached. While 'mobility' and 'immobility' are opposites, as are 'effective' and 'ineffective', the words 'passivity' and 'impassivity' mean the same thing, as do the words 'flammable' and 'inflammable'.
Tired of being described as passive, he decided to be more impassive. He'd be a man of action, or maybe even inaction. What difference could a negated negation make?
Someone or something that bites your ankles.
To a postman, an ankle biter is often known as a dog.
To an adult, an ankle biter may be a toddler.
To hikers, an ankle biter is sometimes a tick.
And so on.
When a man will search for hours to find something that is laying out in the open on a table. Items are often easily found by a women.
Man: "I have been searching for hours for keys."
Woman: "You mean the ones sitting there on the coffee table?"
Man: "Where?"
Woman: "Right there in the middle of that table."
Man: "oh, must have been Male Pattern Blindness"