WARNING
This topic will involve heavy thinking. Feel free to post the usual silly Newgrounds stuff, but if you do want to discuss the topic, it will involve lot's of reading and pondering; There is no TL;DR - it's just not possible to make one accurately enough that describes the topic while not leaving out to much info - or at least not the usual short, tiny TL;DR.
I'm not saying I've created a new paradoxical idea or whatever, but I couldn't find one on Google that matched my scenario.
Yes, I'm aware of other mirror paradoxes; however, this is more of a paradox of a perfect mirror dimension rather than mirrors themselves.
Say, fictionally(?), that what we saw in mirrors truly was a mirror dimension, and that every single thing in that universe - from quarks to atoms to waves of energy - perfectly matched ours in mirrored fashion. It would be impossible to prove this was a mirror dimension using any means because, anything we could possibly throw at it, whether it be gamma radiation or micro particles - would be mirrored in the mirror dimension, and thus would be either perfectly bounced back by the equal force or somehow go through itself and appear on our side and merely seem as if it went through.
For example, say we blasted Beta Radiation at a regular mirror. It would seem as if it went through, just as it would with other objects that are not dense enough. However, the mirror dimension in our dimension would be 2-D, and our dimension would be 2-D in the mirror dimension (also another paradox in mirror dimensions by the way; if both dimensions are 2-D in each others dimensions, how is either 3-D+, to keep things simple), so if the beta radiation did somehow manage to pass through to the mirror dimension, it would come out inversely; in other words, it wouldn't come back at you, but bounce "backwards" from your point of view, thus looking as though it went through the mirror instead of into another dimension.
So unless we had a way to uniquely tag our Beta Radiation particles, not even that would disprove that the mirror is simply a regular reflection and not a view into a perfect mirror dimension - and that's assuming that the tagged mirror dimension particles weren't tagged in the exact same way and are identical to ours in the mirror dimension, since it is a perfect mirror dimension and therefore the exact same actions and physical properties exist there as do here. This would also allow balance in both dimensions, since if the particles that do go through into the mirror dimension were not the same, then it would no longer be a true mirror dimension.
Now, you might ask, why would a mirror dimension even exist or how? Well, it could literally be the passing of time itself that we are viewing in the mirror dimension, and although a perfect copy of our own dimension, it can be offset in physical space between each other by such a small fraction it is almost unmeasurable (also would explain how the particles would be able to travel between the dimensions), and that differential in space would not be possible to measure unless you were outside the affected dimensions (ours and the mirrored one) or possibly through highly advanced math - supposing we would be able to get that far in the first place since we would have no way of being able to prove said mirror dimension other than by means of precisely determining if the particle "went through" the mirror in the exact spot it should have or by an almost unmeasurable offset.
Therefore, with current technology (and perhaps technology for thousands of years), we cannot prove whether what we see is truly a mirror dimension or not, although if we had the technology and math for it, would be possible.
And as mentioned shortly before hand, there is that paradox of Two 3-D planes existing with Two 2-D planes simultaneously if said mirror dimension is real.
P.S.
If these mirror dimensions did exist as a result of the passage of time, it would also explain the infinite reflections seen when placing two mirrors facing each other. It would also mean that we are the past, present, and future all at the same time, and while the reflection is our past or future, the same is true for the mirror dimension.