A commercial aircraft carrying 326 passengers was forced to divert to Shannon, Ireland, after a coffee spill caused two audio control panels to fail.
The flight had been heading from Frankfurt Germany to Cancun, Mexico on February 6.
After the coffee cup was knocked over, the audio control panels became very hot, eventually producing an electrical burning smell and smoke in the cockpit, a UK Air Accidents Investigation Branch (AAIB) report said.
The AAIB did not say what airline was involved in the incident but provided a flight registration number. The flight was registered to Thomas Cook Airlines, according to the UK Civil Aviation Authority.
The pilots had been served coffee without lids, which was common for the route and plane operator, the report said.
It described how most of the coffee fell into the commander's lap, but the small amount that spilt on the audio control panels caused significant disruption to communications.
The first audio control panel malfunctioned immediately. The second one started melting one of its buttons about 20 minutes after the first panel failed.
As the plane diverted to Ireland, the pilots alternately used oxygen masks. But the fumes did not result in any injuries to those on board.
The AAIB report states that the operator now makes sure there are lids on coffee cups and sent a notice to pilots to be careful with liquids.