Reconsider what you have just said, because you are wrong. The phrase is often followed with a presentation of proof of some kind. I know you said he would never show up, but think again, because here he comes.You thought a comeback was impossible? Think again!See also: again, think
ˌthink aˈgain
consider a decision again and perhaps change your idea or intention: I’d advise you to think again before leaving your wife.See also: again, think