“Unwanted surprises”

For the ones in this forum that have been close to a M&A and witnessed these: if and when you have come across unforeseen surprises during the PMI, what did you learn from it? What could you have done differently to not be surprised? What will you change the next time, to not come across such a hidden surprise?
I’d be curious in learning from you on some of the oversights that can happen during a due diligence process, no matter how proper executed it was.