It may seem to us that there are things that no one can be skeptical about. For example, it would seem to be impossible for a person to be skeptical about the fact that they exist. However, even this is possible. We can hold that we are only imagining that we exist and there is no way to prove that we are wrong. Historically, there have been philosophers who have argued that there is no way to truly know anything. These thinkers have also claimed that there is no way to prove that anything we see is real.
We can even take this idea a step further. There have been people who have claimed that we should even be skeptical of the claims of skeptics. These were the Pyrrhonian skeptics like Sextus Empiricus. They said that someone who claimed that nothing could be known was claiming that they know something. This means that, at least to some thinkers, there is absolutely nothing that we can truly claim to know. In this view, we can be skeptical about anything and everything.