Whatever the cause - code, compiler, browser - what you have is a NaN in the wrong place, so the usual techniques apply: Test your code (which usually consists of fairly small functions) using Haskell's excellent testsuites, figure out error location with Debug.Trace, etc.
In the end, this is like asking what happens when the assembler "freaks out" (inserts 0 at a random location), or your architecture is wrong (Pentium FDIV bug): things may become difficult. (There is no type safety to help you on the JS level, much like there is no type safety in machine code. When you insert a wrong value, your code will misbehave in an unpredictable way.)
