The corrcoef function in the nan package uses a numerically unstable method to calculate the correlation coefficients, which can even result in the returned correlation coefficients being outside the range -1 to 1.

Examples:
A) i = 1E7; c = corrcoef(1E5 + (1:i)'/i, 1E5 + sin((1:i)'/i) .* cos((1:i)'/i)) returns 1.0518
B) i = 1E6; c = corrcoef(1E7+(1:i)'/i, 1E7 + sin((1:i)'/i)) returns -1.2129
C) i = 1E6; c = corrcoef(1E7+(1:i)'/i, 1E7 - sin((1:i)'/i)) returns 0.00000 - 0.43355i

By contrast, for the above cases the built-in corrcoef function returns reasonable results of 0.92305, 0.99743, -0.99743 respectively