Lua uses floating point by default, it can't make integer math at all, I made a game using Lua, back then DirectX had a bug where it would switch the floating point mode of the processor without permission, and sometimes the result was some extreme FPU imprecision.
Since Lua uses only the FPU for maths, even for integers, the result was that my game had lots of extremely bizarre results sometimes on Windows (while on Linux and OSX it worked as expected), it happened only once, but I saw 5+6 result in 13...
It was a really hard thing to debug, specially because of the flaming (when I asked about this on Lua IRC and forums, people flamed me endlessy, happily one guy in particular suggested me to check if it was the DX issue, and indeed it was).