This is no time for true confessions of major or even minor problems.

BEST ANSWER:  Again never be negative .

Example:  "I suppose with the benefit of hindsight  you can always find things to do better,  of course, but off the top of my head, I  can't think of anything of major consequence. " 

(If more explanation seems necessary)
Describer a situation that didn't suffer because of you but from external conditions beyond your control.

For example, describe the disappointment you felt with a test campaign, new product launch, merger, etc., which looked promising at first, but led to underwhelming results.  "I wish we could have known at the start what we later found out (about the economy turning, the marketplace changing, etc.), but since we couldn't, we just had to go for it.  And we did learn from it."