For example:

const arr = ["hello", "world"]
for (var i in arr) {
    console.log(arr[i]);
    if (i == 0) arr.splice(i, 1);
}

This should only print "hello", however, we print "hello" and "undefined".

This affects the game notification system of classic and current Cookie Clicker, which checks if the notification has expired in a for/in loop and splices the notification out of the array if it has. If this was a notification in the middle of the array, it will subsequently throw when it reaches the undefined value, which is unexpected.