Descartes' seminal treatise of metaphysics, Discourse on the method, contains several somewhat famous arguments which range from philosophy to sophistry, the best-known of which is "I think, therefore I am" (so well-known that countless people can parrot it without having ever wondered what it means). Many people who've done a bit of reading about Descartes know summaries of his arguments which are paradoxically both accurate and misleading.
Generally, his arguments are a tough slog, largely because of the difference in writing style between then and now, and partially because of his own style, not to mention the subject matter. So often, an argument that takes many pages and veers through a number of corrolary issues will be summarized as a few sentences. This "abstract" of the argument is usually accurate, to the extent that, if you've read the original, you'd have to agree the abstract summarizes it, reflect it, and is really what he argued. But it's also deceptive, because it tends to lead people to imagine that a refutation of the abstract is also a refutation of the argument, when it is usually not quite that easy.
Even if you hear the abstract and then go read the original, your understanding of it is likely to be colored by what you are expecting it to say because of the abstract, and the refutations or analyses you come up with will often be more similar to those envisioned by people who only heard the abstract than by those who read the original work.
The "I think, therefore I am" argument isn't the best example of this. If you take the abstract of that argument, and decide what he's saying, you will probably bring up criticisms of it that are mostly but not perfectly on target. The biggest falling-down point is the question of what "I" represents. Descartes doesn't conclude, with that sentence, that there must exist a human being named Descartes. He only concludes that since the thinking exists, there must be an agent that is doing the thinking, which he calls "I". He concedes that it might not be what he imagines it to be (at least until later in the book). The typical first reaction to this bit of philosophy is to assume he means differently and then refute the assumption. But that's a simple error and not going to confuse anyone.
When you get to the argument for the existence of God, Descartes does a similar thing but the typical reaction to the argument, from people who heard only the abstract, is farther off the mark. The way the abstract was presented to me in a freshman college philosophy course is this. "God is by definition that which is entirely perfect. It is more perfect to exist than to not exist. Ergo, God exists."
This just screams out "sophistry" and begs for refutation. The easiest refutation: "The Wonka Scrumdiddlyumptious bar is, by definition, the perfect chocolate bar. It is more perfect to exist than to not exist. Ergo, The Wonka Scrumdiddlyumptious bar exists." But this really doesn't add up to a refutation of what Descartes argued. Even though the abstract is an accurate summary of Descartes' arguments, it misleads you to a refutation that doesn't apply.
Descartes actually does something similar to the "I think, therefore I am" argument here. "God" is not (at this point) necessarily the Creator, or the Christian god, or even a thinking being. "God" is simply the label for "a perfect thing" he uses for his argument. He does not yet at this point speculate about what God might be, save only the question of whether, whatever it is, it exists, and he concludes it does. Later, he builds on these two facts (that some "I" exists to be doing the thinking, and that some "God" exists that is perfect), and uses them to systematically rebuild all of existence -- as Douglas Adams would put it, to infer the existence of taxes and rice pudding. But his early steps are very basic since he's starting from nothing.
Because he chose heavily loaded words (which would turn out to be well-chosen only later into the argument), taking these arguments in abstract form easily leads to assumptions about what he meant that are not applicable, which cause refutations that are not germane. Refuting the logic of his argument for the existence of God is far from trivial. Not saying it hasn't been done; just that it tends to attract cranks with over-simple refutations the way Einstein attracts cranks who can prove that the speed of light isn't an absolute limit with ideas about spaceships with headlights.