Executable stops working after saving from hex editor?

Question

I've been playing around with binaries and clang. I was playing around and suddenly the executable stopped working for no apparent reason. Here's what I did:

1. Build hello world
   ex: echo -e '#include <stdio.h>\n int main() { puts("Hello"); }' | clang -x c -
2. Use shasum on a.out
3. Run ./a.out to confirm it works
4. Open the binary in VS Code hex editor, change a letter, save
5. Run to confirm it not executing
6. Use shasum to confirm the sum is different
7. Change the letter back
8. Use shasum to confirm the sum is the original value
9. Run ./a.out and see it still fail

Any idea why?