For the Prime Minister, it was his continual failure to answer direct questions.
Mr Johnson is a true pioneer in the field, having skipped Andrew Neil interviews, talked around subjects or simply hid in fridges on his way to the highest office in the land.
Wednesday’s dilemma was being asked to explain his decision to weaken travel restrictions by moving 170 countries or territories to the amber list on Monday.
To great laughter in the Commons, Mr Johnson insisted the UK has “one of the strongest border regimes anywhere in the world", before urging Sir Keir to support the government.
As ever with this Prime Minister, he sees the exchanges as an excuse to ask for validation, rather than an opportunity to explain his actions to the public.
He continued to stress the government had been clear on travel restrictions, quarantine measures and penalties for breaching the rules.
Like a child insisting it believes in fairies, Mr Johnson’s belief on good policy appears to be to simply repeat it until it’s the accepted reality.
Consistently strong polling despite his premiership overseeing one of the highest death tolls in Europe suggests the approach is working.
For Sir Keir, his criticism was held back by his lack of commitment to a policy over the past year.
This isn’t an isolated problem for a leader still trying to discover what he is behind good intentions and a nice haircut.
Later he raised the horrific anti-Semitic incidents in London over the weekend – a valid concern and something worthy of addressing.
However, due to his silence when remaining part of Jeremy Corbyn’s shadow cabinet, the Prime Minister had free reign to almost mock his new found concern as a “U-turn”.
Mr Johnson’s response suggests the issue isn’t taken particularly seriously by his government, but it doesn’t matter.
Sir Keir’s failure to take a stand before has given the Prime Minister the upper ground on the subject.
This again points to the great problem facing the Labour leader, and his party more generally. If you stand for nothing, what will you fall for?