The 'Boom Clap' singer visited the 30-year-old star's home to discuss touring plans, but a heavy evening of drinking the night before led to the alarming incident.
She told Shortlist magazine: "I went to her house, but was so hungover, I had one drink and passed out in her bathroom."
Katy - who is currently rumoured to be dating hit DJ, Diplo - was not scared of the 22-year-old popstar though, and asked the young singer to support her on her 2015 European tour dates.
The support slot was not something which the 'Fancy' singer expected though, admitting: "I'm surprised she asked me."
The star also recently revealed another drunken incident which resulted in a positive outcome, explaining she was inebriated when she wrote 'Kingdom' with Vampire Weekend musician Rostam Batmanglij, which now features on the soundtrack of 'The Hunger Games: Mockingjay - Part 1'.
She told music website Pitchfork: "We went to the Miley Cyrus show in LA and got really wasted. Then we went back to his house and I sat on top of his piano, and we wrote 'Kingdom'. We sung it into his phone.
"I remember thinking in the morning, 'Ugh, this is going to be the worst thing ever,' but it was really good.
"So when Ella [Lorde] reached out to me about the soundtrack, I decided to send her that song even though it's really different than my usual s**t. She was really complimentary about it."