Some things to be wary of:

• the executable comes from the source code you're reading
• the problem must be in this source file
• the problem cannot be in this source file
• code that calls a function never provides unexpected arguments
  (e.g. "this pointer will never be NULL; why would anyone pass a NULL?")
• library functions never return an error status
  (e.g. "malloc will always give the memory I ask for")