One police officer was accused of calling a man "a black bastard". Another was accused of racially abusing a black woman.
The first was let off when his force took his denial at face value, believing it "could not have happened" because the officer was with a journalist from a local newspaper at the time. The second was investigated and sacked.
According to a new report, the crucial difference in the cases was this: the first complaint came from a member of the public, the second came from the officer's police colleagues.
The Independent Police Complaints Commission (IPCC) said police officers take racism complaints much more seriously when they come from their own ranks - more than half of those the police watchdog looked at were upheld, while barely 1 in 10 from the public were.
The discrepancy prompted one MP to warn that the police had to do more to ensure there were no "double standards" when investigating police racism depending who had made the complaint.
In the case where a member of the public complained, the investigator did not speak to any of the nine other officers present, or the journalist, and simply accepted the officer's brief email denying it.
"The investigator presumed that because the media was there, the alleged incident could not have happened," the IPCC report said.
"The officer complained about provided a two-line email that simply denied the allegation. The investigator did not challenge this."
When the other officer was investigated, it revealed he had previously used racist language and even been warned about it by a superior, which he had failed to heed as a "wake-up call".
The officer who presided over the hearing wrote: "I get the definite feel of a man who believes his moral compass is set right and the rest of society is out of kilter.
"I have considered all options in sanction but believe that there is, sadly no place for you in [the force] as I believe you would reoffend."
The IPCC, which this week accused Greater Manchester Police, West Midlands Police and West Yorkshire Police, three of Britain's largest forces, of "not understanding the diverse communities they serve" for their poor complaints handling, said the only discrimination complaints against officers it had seen that reached formal misconduct hearings were complaints from within the police, not from the public.
"This, combined with the number of complaint investigations where the officer was believed rather than the complainant, leads to the inevitable conclusion that an officer is more likely to be believed and taken seriously than a member of the public," its report added.
Dame Anne Owers, the chair of the IPCC, said the findings were "stark", adding: "It is of course welcome that some police officers are themselves prepared to challenge colleagues, but allegations from the public need to be taken
equally seriously and dealt with effectively."
Shadow Policing Minister Jack Dromey said: "All complaints of discrimination should be properly investigated, whether by the public against the police or the police against the police. And there can be no question of double-standards."
The issue was even raised with Nick Clegg on LBC Radio on his weekly Call Clegg show.
He was asked whether the police were still "institutionally racist", an allegation that has dogged British policing since the Stephen Lawrence inquiry in the late 1990s.
Clegg said: "I think a lot of progress has been made since the Stephen Lawrence inquiry, but there is still some way to travel.
"I am sure that the forces identified by the IPCC will ask themselves very searching questions about how to improve their inquiries when very serious allegations like this arise."
The Equality And Human Rights Commission declined to speculate on the roots causes behind the IPCC's findings but said it would review them in detail to "help police deal with discrimination complaints properly and fairly".
Police chief Mike Cunningham, who is the national lead for workforce development, said he recognised the police had "more to do to improve the way complaints from members of the public are dealt with".
He insisted the police service took discrimination seriously, adding six police officers had been sacked across the country in the last six months for breaches of conduct around equality and diversity.
"There is more to do to ensure that our complaints procedures are robust at every level of the service, we will continue to work hard to root out those officers whose conduct and behaviour undermines the actions of the majority of their colleagues," he said, in a statement.
Another senior officer - from one of the three forces the IPCC reported looked at - was not happy with some of its conclusions.
"We are very disappointed with the suggestion that we are 'failing at every stage'," said West Yorkshire Deputy Chief Constable Dee Collins.
"Furthermore, we refute the report's assertion that we 'do not have a good understanding of the communities we serve'."