Ah, the Great Mistake. Very intelligent poll, by the way! Respect to the topic starter is.
In my opinion, it's "none of the above". Basically, i think that the Great Mistake is we. We humans. We ourselves is the mistake. See, humans are intelligent animals. It is in the nature of animals to make a mess. Intellect boosts humans' ability to do things; all things. Including making a mess. Lower-sentience animals (ones without a language and ones with rudimentary language - 1000 "words" or less, approx) lack the ability to transform their environment into "artificial" shape any much. Granted, ants, termites, some apes, some water mammals, some birds and certain other animals - transform their environment to "artificial" a little (nests, using primitive tools, etc). But only human-scale intellect allows to transform the environment on a large scale; for example, during last ~40 years, mankind cut down about 50% of remaining forests on Earth (and surely more even before that).
The great mistake, to me, - is a figurative expression. I don't see anyone (nor any group/country/corporation etc) actually _doing_ that mistake - now or in the future. Instead, we do what we must: we must eat, we must sleep, we must keep our bodies (most of the time) within rather small range of outside temperature. And being animals, we must make love (most of us anyways), have children, and care about them. This all means, we need food, clean water, shelter, medicines, clothes, etc etc. And when it's 7+ billions of us, with every last person needing all those things and more, for decades onward, - this is, inevitably, a huge stress to the Earth biosphere (which is, ultimately, the source of most of those things). And i underline, _needing_ them we are - it's not something we "could" stop getting.
Unavoidable result of the mankind proliferation into industrial ways of life (which in fact is the ultimate step on the "making environment artificial" road), - is massive reduction of natural biosphere (a.k.a. Gaia) of Earth. In simple words - Earth is getting extremely very sick, because of us. This is not a joke; google for "Sixth Great Extinction" - it is going on already as we speak, accelerating.
It's also known as "holocene extinction". It's called "Sixth", because in geologic record, scientists see five Great extinctions of species on Earth in the past. Dozens and hundreds millions years ago, they happened. One of them was about ~96% of species gone (extinct), the remaining 4% greatly reduced in numbers of surviving organisms. What caused them? Big things: asteroid impacts, possibly one or two - huge methane bursts (we're talking hundreds to thousands BILLIONS of tons of methane released short-term), etc. And we humans - are in fact on par, if not worse, with a massive asteroid impact, you see. Whos mistake is that? Noone's... Noone we know, that is. If God exists, - perhaps it is his mistake. But most theistic people will surely mention that we can't know if it is a mistake - perhaps it is a divine plan, a way to find out who's worthy of eternal life, and who's not, etc. Atheistic people will probably agree with me when i say that this unfortunate dual nature of humans (animal and sapient) - is only a manifestation, ultimately, of laws of physics, chemistry, biology, which, given enough time, eventually led (through evolution) to appearance of sapient humans, and their deeds. To clarify - myself, i'm not "theistic" nor "atheistic" - i just remain unsure. Agnostic, sort of. I say - "may be there is God; may be there is not - i do not know".
Some will ask me: "what about freedom of choice? Can't we humans prevent planetary disaster here on Earth, if we really want to do so?". The answer is: freedom of choice, sadly, is weaker force than human instincts and desires, in most of us. Only few humans develop their intellect high enough to be able to "override" their _needs_ and desires which are based on their instincts - and sadly, for most of those brave souls, it doesn't end well. Evolution itself stimulates proliferation of self-preservation instinct, breeding instincts, feeding intincts, etc - those individuals who follow those intincts have higher chances to make kids, and this means, natural selection favors them.
Many of us here - are in part, or even mostly, able to actually behave against our own survival instincts when our mind tells us it's the thing to do. For now, that is. But please don't think whole manking is so; people are different. And even more important - people's circumstances are different. We might dream all day how in the future mankind will change its ways, will stop cutting down forests, killing lots of species with our chemical, radioactive and other kinds of pollution, etc - but how many of us will keep dreaming about it, if we are put into "poor man of a third country" environment? Hungry worker somewhere in Bangladash, being paid an equivalent of a few dollars for a month of hard work, is not likely to care (any much, in practice) about well-being of Earth biosphere; his hungry kids and starving wife occupy his thoughts much more, i think. And he's not alone; most people in the world are in similar environament/situation all the time. Trust me, even few days of empty stomach changes a man VERY much, and Maslow's pyramid is not just a pretty picture - its meaning is clear and obvious (to me, at least): physiologycal and safety needs - take absolute priority...
Multiply that by 7+ billions people, several decades of keeping them alive and quite many of them (so-called "1st world") - at least relatively "thriving", and consider how you could do so on finite Earth with existing real modern-day technologies - and you will realize: we humans are killing life on Earth, and this will hit us back right in the face, during this century, and kill most of us humans - because our food, most of clean water, most of medicines and even habitable climate - we get from Earth biosphere, i.e. from other life forms. Unlike simple organisms like green algae in the ocean, we can't exist purely on sunlight and few common elements abundant in the sea water...
If that's not a "Great Mistake", then what is?
P.S. I've wrote somewhere in the internets about it already few months ago, but can't find where. Sigh...
P.P.S. If our leaders are stupid enough to do all-out nuclear war, using most of existing nuclear weapons, - then yes, it'd be very proper MISTAKE indeed; but i really doubt any space ships would be able to leave Earth in this case - even in purely fictional universe like Civ:BE. Like Einstein's popular quote says, "4th world war will be fought with sticks and stones" - after nuclear world war, mankind would be lucky to survive _at all_, barely, as a few tribes of cavemen in remote locations...