On 10 June 2022, on reaching the planned FL330 cruise altitude abeam Meekatharra, a Boeing 737-400 crew observed a 340 feet difference between the two primary altimeter readings. The crew did not advise ATC of the breach of RVSM separation minima but after identifying which instrument was in error, they returned uneventfully to their departure point, Perth, where an engineering inspection found residue on all four electrically-heated pitot/static probes. It was found that a few days earlier, an engine ground run without first removing non-standard plastic probe covers had contaminated the probes with cleaning performed prior to release to service.