|Bryan Cranston & Aaron Paul|
Aaron then realised Bryan - who stars as chemistry teacher Walter White - had revealed his butt to the camera and crew in the middle of their poignant moment.
He told Shortlist magazine: "I remember the final scene that he and I were shooting together.
"It was this serious moment, I'm talking to him and I hear everyone laughing outside of the set and I'm like, 'Why is everyone laughing?'
"Then I realise. He turns away from me and starts walking and I can see that his ass is out. Right in front of the camera."
The programme will come to an end next month and Bryan's prank comes as a stark contrast to the tone Aaron has recently hinted at for the final series.
He previously said: "Each season of Breaking Bad gets progressively darker and this final season, each episode just gets more and more intense.
"And just hold on because it's about to get really, really, really crazy."