While tuning some code, the test programme suddenly started producing stack overflows. Reverting the code to a previous version did not revert that behaviour, code that previously produced a well-behaved binary now produced stack overflowing ones. But only with ghc-7.0.1, not with ghc-6.12.3 and ghc-6.12.1, and when compiled with optimisations (both, -O and -O2) and without profiling. A HEAD from October and a freshly darcs pulled HEAD displayed the same behaviour as 7.0.1. At some point I tried compiling with -fno-strictness (+ -O or -O2) which again produced well-behaved albeit somewhat slower binaries. An hour or so later, I again compile with -O2 without -fno-strictness to see whether the Core revealed something and, as suddenly as the stack overflows started, they disappeared, for the time being, I consistently get well-behaved binaries. A run of an overflowing binary with -hT -K32M produced a triangular graph with > 10MB Blackhole allocation and > 30MB TSO allocation in the peak. Obviously the behaviour is not reproducible, nevertheless, perhaps somebody has an idea what went on. Or should I attribute it to cosmic rays? Cheers, Daniel