Detective Craig Appel testified Monday prosecutors notified police in June the court order they used to seize Holmes' bank records from Point Loma Credit Union in California didn't have a judge's signature on it.
Detective Mark Yacano, who drafted the order in July 2012, was working with a detective in San Diego to have a California judge sign off on the paperwork, The Denver Post reported.
The California detective submitted the paperwork to the credit union, thinking it was complete after a notary signed in the spot where the judge was supposed to sign.
Yacano said he received the bank records anyway in August 2012. When police realized the mistake in June, officers resubmitted the paperwork to a California judge for a signature.
Defense attorneys said the bank records should be inadmissible because of the improper seizure.
"They don't get a do-over on this one," defense attorney Kristen Nelson said.
Prosecutors said the bank records didn't give investigators any information they didn't already know.
Holmes faces multiple first-degree murder charges in the massacre that left a dozen people dead and 70 injured at a theater where a Batman movie was being shown. Holmes has pleaded not guilty by reason of insanity.