[nicwilson]

One of my worst ones was a compiler bug for a PLC which would cause a floating point operation that underflowed to become NaN instead of 0.0 (which is very common if you are writing set-point tracking code!) and then throw out the loop, so the controller would reach set point and then slowly start drifting as is accumulated error, but only for the rest of the "line"* of that code. So if you split your calculations across multiple lines then you were fine, but if you tried to group your operations sensibly then it no linger worked.

PLCs do the loop for you, you only write the body

*it was some bastardised version of ladder logic (itself a bastard representation of code) with functional blocks, so "line" = rung. I no longer work with PLCs.