"Unfortunately sometimes these things can happen, " Massa added.

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It is an embarrassment for TfL and Boris Johnson, although the reality is these things do sometimes happen with new transport systems.

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With extremely sophisticated attackers and the way we must run our business in terms of the extension of trust, these things happen sometimes.

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"You don't want this to happen, but we're in the country and sometimes these things do, " he said.

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