There's been plenty of talk at this point about the ending possibly being a hallucination and/or an indoctrination attempt by the Reapers against Shepard, but considering it's usually presented in bits and pieces, it's very easy to discount as nonsense and wishful thinking. However, in my opinion, there's a pretty solid (and mounting) body of evidence. Here's some found so far, in a condensed form:

Premise: Shepard is being slowly indoctrinated, and may be hallucinating everything after being hit by Harbinger's attack while running toward the Citadel transport beam.

Major in-universe evidence is as follows:

• The Starchild is part of the Reapers (self-admitted) and not a reliable character.

• When presenting the three solutions to you, Destruction is presented first, and the negatives very clearly emphasized: Shepard will die, the Geth will die, EDI will die, and the synthetic singularity is inevitable and all organic life will be extinguished forever.

• Control represents the motives of The Illusive Man, who thought he could manipulate the Reapers. It is known by now that TIM was slowly, and without his knowledge, indoctrinated. The Starchild tells you that you can control the Reapers when TIM could not, but the Starchild is not reliable.

• Synthesis represents the motives of Saren, who wanted to subjugate himself to the Reapers and work with them. He also, as an organic, worked with the Geth to this end. While it wasn't a literal merging of synthetics and organics he wanted, it was a synthesis of sorts. Saren is also known to have been very slowly indoctrinated when he believed himself in control.

• Destruction represents Shepard's original motive, and not that of either of the known-indoctrinated enemies. It is also the only one of the three that results in the destruction of the Reapers, rather than compromise.

• Shepard only survives with a lot of war assets, and only when choosing the Destruction ending. The pile of rubble he is seen in is very clearly stone, like in London. There is no stone on the Citadel and he would not have survived the destruction of the Citadel regardless.

• In choosing Destruction, Shepard is rebelling against the indoctrination attempt, recovering his mind, and reviving in London after being hit. In choosing either of the other two, Shepard is submitting to indoctrination just like TIM and Saren, and dies in the rubble. Sufficient war assets are required to give him the time he needs to come to his senses.

Minor in-universe evidence:

• Shepard has spent a lot of time around Reapers, Reaper-controlled areas, and Reaper tech. Not as much as TIM or Saren, but he's hardly under full control like those two; it's a much more subtle, suggestion-based indoctrination.

• No one ever reacts to the child but Shepard. He is alone in the courtyard at the start, Anderson never sees or hears him in the vent, and no one notices or helps him into the extraction shuttle.

• The ME3 codex entry for indoctrination specifically refers to the sight of ghostly figures. The Starchild's choice of form is odd to begin with, and he's clearly ghostly.

• Shepard cannot argue with the Starchild beyond single half-hearted comments that are immediately brushed off. This is different than every other conversation in the entire series.

• The Starchild's first words without enough war assets and with enough war assets for him to survive the Destruction choice. An odd difference.

• The Starchild's voice sounds like it simply has reverb, but it's actually Mark Meer and Jennifer Hale (male and female Shepard) both repeating the lines at the same time. This suggests the words are, to some extent, in Shepard's head.

Metagame and player-centric evidence:

• "Shepard" doesn't have free will, in that he's controlled by the player. At the end of the day, Shepard can say whatever he wants in cinematics, but the player is making his choices and controlling where he moves and what he shoots. Any attempted show of indoctrination would be strange; the player wouldn't willingly go over to the Reapers, but Shepard doing so on his own suddenly takes control out of the player's hands.

• Any real attempt to show the player indoctrination has to be just like TIM and Saren saw it: the player feels like they have all the information, feels like any option is available to them, and they reason through it and come to their own conclusions willingly (or so they think).

• Blue and red are very clearly defined colors in this series, as Paragon and Renegade. Control is colored blue and represented by TIM; Destruction is colored red and represented by Anderson. To anyone familiar with the series, this should feel backward. Things aren't being presented as they should be.

Some bits of evidence are stronger than others, I admit freely, and the entire thing remains a long shot. But the endings as they stand are simply strange, in every way, and upon even a shallow examination with a critical eye there are some weird things to explain. Even if Bioware never admits anything either way, all of this is plausible enough in my mind that I can convince myself it's true, and well, it makes me feel better.