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Sunday 6 May 2018 at 12:39
Do a "hard reset". Competely remove all batteries. If using NVRAM, remove the socketed chip. Make sure the board is discharged, meaning give time before a restart, around 30 minutes. Make shows the time/date error. Memory errors do happen. Software resets can retain the error. After this step...
Source: http://pinside.com/pinball/forum/topic/monster-bash-stuck-on-free-play#post-4382156