The trial of a twisted Brooklyn ex-con who allegedly shot his parole officer — and admitted that the DA had a slam-dunk case against him — was tossed yesterday on a technicality “the size of a thumbnail.”

Brooklyn Supreme Court Judge Wayne Ozzi declared a mistrial after the prosecution gave the defense only a photo “the size of a thumbnail’’ of a fingerprint taken from the 9mm gun that Robert “Poison’’ Morales allegedly used to wound parole officer Samuel Salters in 2010, sources said.

Morales’ lawyer needed a digital copy to be able to blow it up to analyze it properly, said a source close to the case.

“I want my client to have a fair trial, which includes him being given the evidence that is required by law to be turned over to us,” said his lawyer, Kenneth Perry.

The judge KO’d the trial as it neared summations.