What are ways a debug statement could "fix" bugs in a program?

Question

In a recent interview, I was asked the following question:

You have a bug in your program, after attempting to debug it by inserting statements like printf, console.log, System.out.println, echo, etc, the bug disappears. How can this happen?

I responded with answers like the following:

• You have something with side effects in the print statement, eg: System.out.println(foo.somethingWithSideEffects())
• Adding printf changes the memory layout of the program, therefore it could cover adjacent memory and prevent crashes
• Undefined Behavior in native code (like uninitialized values, buffer overruns, sequence points, etc)

The interviewer said those aren't the only ways that this could happen, and I couldn't think of any other ways simply adding a printf, etc could "fix" a bug in a program.

What other things could cause this to happen?

Answer 1

The biggest thing that comes to mind is that putting debugging code in can change the timing of the code, which can matter if there is a race condition in the code being debugged. It can be very frustrating to try to debug race conditions that disappear when inspected like this.

Answer 2

That could be happen because of memory overflowing , or there could be system interrupt while the program running.If you cannot really attach the debug so you may write eventlogs but it should be the last way to do i think