Johnson has sought to justify the bill, which will give his government the power to double back on part of the Brexit agreement they made with the EU (and in doing so, potentially break international law), by claiming it will provide vital trading safeguards for Northern Ireland if negotiations break down.
But Miliband cast doubt on whether or not this is actually true: he invited Johnson to point out specific safeguards in his bill, which Johnson failed to do. Instead, he appeared visibly shaken by Miliband's speech.
And obviously he had time to check his phone as well.
We do wonder what he was looking at? One social media user offered up a suggestion with this mock-up image:
We would all love to know what was so interesting though.