It is the interest-free loan that has turned out to be far from interest-free. Labour may have benefited financially from Owen Glenn's generosity. But the party is sure as heck paying an exorbitant rate of interest in political terms.
Despite getting the $100,000 interest-free loan on top of $500,000 in donations from the expat businessman, Labour must have been asking itself in recent days whether its association with the billionaire philanthropist has been worth the hassle.
The party will have been pondering that question even more intently following the drubbing it received at the hands of National in Parliament yesterday.
The trouncing was almost complete before Justice Minister Annette King described Bill English as "Mr Nasty". But that sealed victory for the Opposition.
When a minister as experienced and normally dominant on the parliamentary stage as King resorts to name-calling to avoid answering a question from an opponent, that is as close to an admission of defeat as you are going to get.
National's deputy leader had been repeatedly asking her whether she would be investigating whether her party had been complying with electoral law with respect to such loans. English got under her skin. She committed the parliamentary no-no of showing it.
English's persistence made it a grand slam for National after John Key also got the better of the Prime Minister on the subject of Glenn, his donations, the loan, his New Year's honour, his claim (subsequently withdrawn) that he was offered a Cabinet post and, in the latest bizarre twist, his possible appointment as honorary consul in Monaco, his official home.
Key cut straight to the chase, asking Helen Clark if she had been aware of the loan when the Cabinet honours committee recommended Glenn for an Order of Merit.
The Prime Minister confirmed she had been, "but, of course, the honour was made irrespective of such factors".
Key then wanted to know why she had not corrected Labour Party president Mike Williams for saying - after the honour was made public in January - that Glenn had not made a donation to the Labour Party since 2005, given such interest-free loans counted as a donation.
Clark retorted that she had been overseas "and had no knowledge of it at the time". She similarly proclaimed ignorance as to how the Government had come to be considering Glenn's appointment as honorary consul.
Key then pondered why the Prime Minister struggled to either remember or reconcile things when Glenn's memory seemed to be intact.
Clark struck back, declaring that was a "bit rich" coming from the man who could not remember opening the email he knew he had received from the Exclusive Brethren.
It was a quick-witted response, backed up by her assertion that the only thing looking murky was the National Party scurrying around its anonymous donors, "such as the Waitemata Trust run by Mr Bob Brown, business partner of Murray McCully", before the deadline for new rules on party funding came into force.
An exchange between English and Labour's Phil Goff summed up the clash.
"Quit while you're ahead," interjected Goff. "Quit while you're behind, Phil," English shot back.