What were the Synthetics the Child was talking about?

Mass effect 3 ending spoilers here.

So the kid in the ending states that the whole Reapers thing is to cull Organic life to save the Galaxy from Synthetics taking over. But until that point there's no indication that Synethics actually do this except for the Reapers (Ironic) that I'm aware of. I didn't play ME 1 or 2 however. The Geth were proven to be peaceful and acted defensively when you go through the Quarian Homeworld part, unless it was a fabrication.

So...what evidence exists of Synthetics actually being bad, or is this (another) plot hole?

The way the child phrases things, it's not an issue of "who?", but an issue of "we can't take that chance".

Back before the Reapers, presumably, some civilization built a synthetic civilization, and it went rogue. The response to this was to prevent any civilization from ever advancing far enough to build a synthetic civilization again.

The Reaper Child isn't looking at any one thing in particular and saying, "That's bad, we should kill it", but rather, "Everything has the potential to do bad things, so we should preemptively kill it".

The problem in many minds, is that evidence in the Mass Effect series points to the opposite - the fact that the Quarian / Geth resolution can even exist seems to fly right above the head of our spectral reaper-child. Not to mention the literal anthropomorphization of EDI, and her interactions with Shepard and joker. The only evidence the player has had up until this point of a synthetic enemy threat, is the Reapers themselves.

The only evidence that the Catalyst's final solution is necessary... was engendered by the Catalyst itself.

Or, in Xzibit's words...

It physically pains me to type this canon.

The Catalyst was making a cold calculated action when it created the cycle. It decided that organic life was valuable and should be preserved, but organic life would eventually create synthetic life in the course of it's development. If organic life created synthetic life on it's own terms, this could lead to chaos and the destruction of all organic life. Not a certainty, but a possibility given the chaotic nature of organics.

The solution that the catalyst came up with was sort of a controlled burn of organic life. It created the reapers to cull the advanced civilizations that could create its own synthetic life and preserve it as reapers. This allowed something of the old civilizations to live on, and new civilizations could prosper.

It's a rational machine mind imposing order on an irrational organic system. Like when we build a zoo and mimic animals' natural habitats in little managed pieces. You have lions and gazelles within 20 yards of each other, but you keep the lions from eating the gazells. The natural order is for the lions to eat the gazells, but, if you do, you have no more gazells.

As anyone who's seen or read Jurassic Park knows, you can't impose order on chaos forever. Organic life was able to overcome the reapers by progressively constructing the Crucible over countless cycles. Shepard is given the choice of how to end the cycle, but the choices all require a great sacrifice.

I think people don't like the ending because they where expecting a space opera ending with dancing ewocks and force ghosts of all you fallen crew mates nodding approvingly. Instead we got a more typical literary sci-fi ending.

Let's face it, the real ending has generated far more thought and discussion than it would have if the ending was Shep and Liara sipping margareetas on a beach.