Recently I had to map an array of strings which represent numbers into actual numbers. Because it looked nicer, instead of passing an anonymous function which calls parseInt, I passed parseInt directly.

['4', '2', '101', '93', /* ... */].map(parseInt);

When I logged the result, some numbers ended up as NaN.

I forgot that parseInt has a second optional argument, so Array.map passed a radix to parseInt which led to parsing numbers in incorrect base.