MPs who wrongly claimed taxpayer-funded expenses for websites featuring party political logos were let off partly because of a watchdog's lax monitoring.
Details of 21 investigations into alleged abuses of taxpayer-funded expenses have been published after a climbdown by the Independent Parliamentary Standards Authority (Ipsa).
All but one of the cases, which numbered former Cabinet minister Alan Johnson among those investigated, involved illegitimate claims for public cash for websites.
Ipsa's compliance officer ruled that they should be spared paying back the cash because the breaches were relatively minor and the watchdog was "in part at fault". Officials should have spotted the infringements and blocked the payments in the first place, Martyn Taylor said, ruling that MPs who had already coughed up should get their cheques back.
Ipsa is also consulting on changes to the rules over when MPs under investigation should be named - including removing the discretion given to the compliance officer over individual cases.
Existing guidelines suggest politicians should be identified when a formal investigation is launched, but no details had been released since Ipsa took over running the system 17 months ago. The then-compliance officer Luke March said in July that naming MPs could be "unfair" and suggested there should be no publicity at all if they were cleared.
Within days of making the comments - which raised the prospect that the new regime could be less transparent than the old discredited one - Mr March resigned. Mr Taylor, previously head of governance at the watchdog, has been filling in until a permanent replacement is recruited.
In its consultation document, Ipsa said that keeping the names of MPs under investigation secret until it concludes would "respect the need for transparency and prevent speculation".
While the arguments were "finely balanced", revealing politicians' identities at the launch of a full inquiry risked them being "accused of wrongful expense claims before a full and fair investigation is completed", it said.
A spokesman said procedures had been tightened to prevent a repeat of the website breaches.