I think it was...

...in Game Change, the book about the 2008 presidential election by John Heilemann and Mark Halperin, that I read about candidate Obama's reaction to the sudden and meteoric rise of John McCain's newly-chosen running mate. While his staff was understandably concerned, Obama remained calm. His response was something to the effect that, "The national stage is unlike anything you've ever seen before and requires a steep learning curve. I know -- I've been running for president for about a year and a half and I'm still getting used to it."

James Fallows, who once worked as President Carter's chief speechwriter, wrote at the time, "My Prediction about Sarah Palin":

Unless you have seen it first first-hand, as part of the press scrum or as a campaign staffer, it is almost impossible to imagine how grueling the process of running for national office is. Everybody gets exhausted. The candidates have to answer questions and offer views roughly 18 hours a day, and any misstatement on any topic can get them in trouble.
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Barack Obama is a quick study and has been campaigning very hard for 18 months. But this summer, when he tried to offer a reassuring message about his commitment to Israeli security with his AIPAC speech, he made a rookie error by getting the standard phraseology slightly wrong.

Let's assume that Sarah Palin is exactly as smart and disciplined as Barack Obama. But instead of the year and a half of nonstop campaigning he has behind him, and Joe Biden's even longer toughening-up process, she comes into the most intense period of the highest stakes campaign with absolutely zero warmup or preparation. If she has ever addressed an international issue, there's no evidence of it in internet-land.

The smartest person in the world could not prepare quickly enough to know the pitfalls, and to sound confident while doing so, on all the issues she will be forced to address. This is long before she gets to a debate with Biden; it's what the press is going to start out looking for.

So the prediction is: unavoidable gaffes. (My emphasis.)

That was in August, 2008. Yesterday, Fallows wrote, "3 Points on Rick Perry":

Just after Sarah Palin was nominated three years ago, I argued that anyone who moves all at once from state-level to national-level politics is going to be shocked by the greater intensity of the scrutiny and the broader range of expertise called for. Therefore that person is destined to make mistakes; the question is how bad they will be. For Palin, they showed up in her disastrous first few interviews, especially with Katie Couric. Perry is getting his own introduction to this principle just now. (My emphasis.)