The Police Ombudsman is suspicious about how the informant known by the pseudonym "Kevin Fulton" was at various times described as "reliable" and "unreliable" by his RUC handlers.
However, both Garda and PSNI sources say it is common for informants to have highly varied performance records.
The world of informant-handling is a murky one. At times informants can give highly accurate information - for which they will expect to be well paid - but the same persons may also present flawed information, and even make things up.
The informant identified as "Kevin Fulton" is known to be somewhat unstable. But he has been close to figures involved in the "Real IRA" and previously in the Provisional IRA.
The Ombudsman, Mrs Nuala O'Loan, questions how the RUC can, at various times in "Fulton's" career, describe him as "reliable", even an "A1" source, and then as unreliable and even a "Walter Mitty".
However, police sources experienced in handling informants say these varying assessments are commonplace when dealing with informants, particularly those motivated solely by money. It is well known that such informants will make up details or embellish information when they have run out of money. When caught lying or embellishing they are duly admonished and their payments are curtailed.
However, they are encouraged to come up with better, reliable information for which they will be rewarded.
The issue of having informants inside terrorist organisations arose in the US in the aftermath of the September 11th attacks. The failure to have "human" intelligence sources - which will inevitably include figures who might sometimes be deemed unreliable - is now regarded as one of the main intelligence failures of the US security agencies.
As a result, it is likely security agencies around the world will become more reliant on human forms of intelligence gathering, rather than much more expensive and less effective forms such as electronic eavesdropping.