The intense moment when there are thousands of people eager to use a new website you developed and there are still bugs and rough edges but the launch time is in 2hrs.
Needless to say I'm pretty nervous.

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?