It has been suggested that bacteria transformed with pBlueScript vector, containing an insert in the middle of the lacZ' gene, can still give blue colour on X-gal, if the insert is small and ligated in-frame with the lacZ' gene (source: my course, ResearchGate).
I don't understand how it is possible that translation doesn't end at the stop codon encoded by the insert. Is this something to do with leaky termination? Moreover, even if leaky termination was possible, this would mean that there is ALWAYS a 1 in 3 chance of the insert being in-frame.