So I thought I'd weigh in here to explain how these keyboards work:
This is the contactor array of a MacBook Pro 2011 13" . You're looking at it from back to front. Those round dots are where the silicone rubber "cones" cause the front array to make contact with the back array, telling the computer which key has been pushed. The width of these (I think deposited silver ) traces is about 0.5 mm.
Now you're only looking at the just the front array. You can see that the contact pads are connected in series. A break anywhere in the trace connecting them would cause the "downstream" keys to stop working.
So here you see a site where galvanic corrosion occurred. I know that this particular MBP was exposed to water. So I bought it very cheaply. I was able to clean and completely dry out the logic board and the MBP worked fine for a while but quickly the Q - W - E - R -T - Y - P keys got wonky then completely stopped working. I bought a replacement keyboard for cheap but then found out that it's held to the front frame with around 80 tiny screws. So I was using it with a BlueTooth keyboard. Recently I bit the bullet and for $69 bought an entire top case for my machine. Putting it in meant that the everything had to come out of and off the top case.
So you can see from the amount of corrosion, it doesn't take much to cause a failure. In this case a drop of liquid less than 0.5 mm would, over time with galvanic action, cause a failure. There was a post about someone who had their MBP in a leather case and when they opened it up there was a lot of moisture on it. It was winter where he lived and we figured it was condensation. So even without spills this could happen.
When I took this keyboard apart I was shocked to find how much crap was trapped under and inside the keyboard. Especially since I pride myself in never having any food/drink near my "precious!"
