The root cause of the error is that the index `n*10000` in the `x`
array access goes out of bounds of the array. This results in a
segmentation fault or memory access violation.

To fix the error, we can either increase the size of the `x` array or
limit the value of `n` passed to `foo` so that the index stays within
bounds of the array. For example, we can modify the function `foo` to
check if the index is within bounds before accessing the array, and
handle the error appropriately. Here is one possible fix:

```
void foo(int n, float b) {
  int index = n * 10000;
  if (index >= sizeof(x)/sizeof(x[0])) {
    cerr << "Error: index out of bounds" << endl;
    return;
  }
  cout << "TEST " << x[index] << endl;
}
```

This fix checks if the computed index is greater than or equal to the
size of the `x` array, and prints an error message to `cerr` if so. It
also returns early from the function to avoid accessing the array out
of bounds.