Can't remember I'd have to look back at threads. I know it happened to Adam too. I found the pic of the what (both bolts) had remained in the block. They don't look too long to have bottomed out though.
EDIT - Had a thought - I think one of the bolt holes in the DET block isn't used for the S13 brackets, but is for the S12 brackets. Maybe the thread isn't cut as deep as on the S12 blocks?