For example, a function might be inlined at some call sites and not inlined at others. Inlined versions might carry 80 bits through the result, but non-inlined version are truncated to 64. So in:
double foo(double x) { return x*3.3; }
double bar1(double x) { return foo(x); }
double bar2(double x) { return foo(x); }
void t_bar(double x) { assert(bar1(x) == bar2(x)); }