For me, there s not one particular sentence where "it falls apart". ME is weird in that - beginning with ME2 - the series becomes very inconsistent in it's plot quality. While smaller arcs and character interactions are great until the end, the series does continue to throw monkey wrenches into the plot wheel at regular intervals. I mean, it's not like the ending disaster is based only on the last 20 minutes. ME2's stubborn refusal to develop the main plotline of the reapers, the treatment of the Crucible throughout ME3, the "development" of Cerberus over the course of the trilogy, all of these points as well as many others contributed to a situation where a truly satisfying ending, that would really tie up the story was pretty much out of the question.
The weird thing is that in between all of these unfathomable decisions, the writers did a really good job of keeping us on the edge of our seats and bringing great characters to life. I believe that this weird combination of the good and the bad is what ultimately spawned the ending controversy. Without the good parts, people would probably never have cared enough to give a dam in the end. Without the latter, we'd be happily celebrating the awesomeness of the series (which a lot of people do).
So no, I can't name the one sentence where it fell apart, IMO, the reasons were distributed over the course of ME2 and 3.