You can call it hindsight bias if you want. What you seem to be saying is that your heuristics would have told you that Javice had no nefarious intent based on

You can call it hindsight bias if you want. What you seem to be saying is that your heuristics would have told you that Javice had no nefarious intent based on that exchange, and you would of course have been wrong. My heuristics are very different. It is very obvious to me that the extra $5k was "conditional" because if Scientist-1 hadn't sent back an invoice for $18,000, he wouldn't have gotten paid $18,000. I have never heard of a company giving a 40% "tip" after the fact to a contractor who was already billing $600/hour. It seems like Scientist-1 did not question this tip because it didn't benefit him to do so. He didn't suspect anything because he was motivated against suspicion. That's a lack of integrity.