Anyone who has ever done live theater is familiar with the brutal drudgery of learning lines. Nina Metz, writing in The Chicago Tribune, has an article allegedly on the techniques actors use for learning lines, though in point of fact, despite certain tricks that may be helpful, it's mostly just brute force and repetition.
She makes one remark that struck me as odd:
In my 15 years reviewing theater, I have no clear memories of an actor noticeably whiffing on a line (the term of art when the mind goes blank is to "go up" on a line), but it happens, and fudging through those moments is part of the job as well. Matthew Broderick ran into trouble in 2009 when he starred off-Broadway in a new play from Kenneth Lonergan called "Starry Messenger." The script was tweaked during previews, and Broderick "called out for lines multiple times," according to The New York Times.
Of course she admits that she herself can't even memorize a sentence, so maybe she really doesn't know.
My experience is much more with amateur theater than professional, but I can identify dozens of times when I've seen an actor go up on his or her lines. Few virtual experiences are more excruciating for me than watching the only actor on stage forget what they need to say next. Of course, if there are other actors on stage they can usually cover up the damage by simply going on, or even by whispering the next line to the actor in question - and yes, actors need to learn not only their own lines, but also those of the other actors on stage with them. There is a reason all actors study improv.
Sometimes the results can be hilarious. My favorite example was the time one of my fellow actors skipped a full three pages ahead in the script. There were several of us on the stage, so the rest of us carried on from there. Until one stickler skipped back to the original point of departure. At that point, there was nothing to do but do the skipped part followed by repeating the two or three pages of dialog already done.
Guns that don't fire, matches that don't light, or fell out of somebody's pocket, and clothes changes that the stage hands left in the wrong place are all part of live theater. My sympathies to the critic who only saw productions too perfect to include any of the above. Or was too clueless to notice them.