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The lawyer who represented Brendan Dassey says he messed up by allowing his client to meet with cops when he wasn't present, and the reason he was a no show is shocking.

Len Kachinsky appeared on "TMZ Live" Tuesday and had a hard time explaining why his PI strong-armed his 16-year-old client into signing a murder confession. He had an even harder time explaining why he then gave his client up to the cops.