Narrative:

After initial contact with ground pushback crew via interphone, asked first officer if we were cleared to push back. First officer replied 'yes.' upon completion of pushback, ground control advised we were not cleared. Ground did say it was ok though. Note: first officer was new hire.

Google
 

Original NASA ASRS Text

Title: A NEW FO INDICATED THE ACFT WAS CLRED FOR THE PUSHBACK WHEN IT HAD NOT BEEN GIVEN A CLRNC AT BWI.

Narrative: AFTER INITIAL CONTACT WITH GND PUSHBACK CREW VIA INTERPHONE, ASKED FO IF WE WERE CLRED TO PUSH BACK. FO REPLIED 'YES.' UPON COMPLETION OF PUSHBACK, GND CTL ADVISED WE WERE NOT CLRED. GND DID SAY IT WAS OK THOUGH. NOTE: FO WAS NEW HIRE.

Data retrieved from NASA's ASRS site as of July 2007 and automatically converted to unabbreviated mixed upper/lowercase text. This report is for informational purposes with no guarantee of accuracy. See NASA's ASRS site for official report.