Multilayer boards are simply very thin double sided and single sided boards stuck together, then through hole plating applied.
When the very first Pace Digital satellite receivers came out (for the South African market) they had sudden massive failures a few months into their use. The reason was a mistake by the PCB manufacturer, nor washing the boards properly before fastening them together. This left traces of etchant between the layers, which over time ate away the required tracks on that layer. Pace managed to find out which tracks were most commonly failing, and called in all their suitable empoyees to mod the boards, hardwiring the potentially faulty connections. If this didn't cure the board, they were disposed of - they didn't have the time to do any more fault finding on them, with literally thousands of receivers failing within a few weeks.