When it comes to nervewrecking situations, what I've encountered so far is a pull request for which the 2 reviewers had opposite opinions.
Something like this:
* opens pull request *
reviewer 1: "update using approach A"
* updates code *
reviewer 2: "this is wrong, change to approach B"
* updates code again *
reviewer 1: "this isn't what i asked for, i'm rejecting your PR"
Oh, also, each of them had their own set of coding standards.

What was your most challenging situation when writing code?