How we change what others think, feel, believe and do
Person A makes a claim. Person B asserts that person A's previous claims or actions are inconsistent with person A's claim. Therefore person A's claim is false.
Thus the present is rubbished by dragging up the past.
You say you are against the war, yet you voted for it.
He wants to re-join the club, yet he left it of his own free will last year.
Just because what is said now does not align with the past, it does not necessarily mean that it is wrong (it could be that the past was wrong, or that circumstances have changed). There are many times in the past that the person could have produced contradictions to the single point being made -- we are not fully rational beings and do contradict ourselves on many occasions.
Personal inconsistency sends mixed messages and will normally decrease trust. When a person claims that another person is sending mixed messages, they are very close to calling a person a liar. If the accusation stands, then everyone who hears is likely to reduce their trust in the accused person.
Personal inconsistency is quite different from logical inconsistency, which is about inconsistency within an the logic of an argument.
Also known as
Ad Hominem Tu Quoque ('tu quoque' means 'you too'), You Too Fallacy