A clerical error? Oh, no - the truth is far more absurd.
(Hold on, folks - this requires a bit of background.)
In a nutshell, since the late 19th Century, it had been suspected that there was a ninth planet, based on apparent irregularities in the orbit of Uranus. This as-yet-hypothetical planet, whose gravitational influence would have accounted for those irregularities, was termed “Planet X”.
The trouble is, nobody could find the thing, no matter how hard they looked. That seemed to have changed in 1930, when a new moving object was finally detected on the outskirts of the Solar system. When word of this discovery got out, the media declared that Planet X had been found, and the object was subsequently named “Pluto”.
However, there was a problem with the newly dubbed Pluto: its faint albedo and lack of a visible disk suggested that it was much too small to be Planet X. In fact, while school textbooks treated the matter as resolved, the truth of the matter is that we had no idea what Pluto was - we didn’t even know for sure whether it was a planet at all, much less that it was Planet X. Though little reported-on by the mainstream press, the search continued.
It wasn’t until 1992 that data from the Voyager flyby of Neptune revealed that prior estimates of the masses of the outer planets had been slightly out of whack. With the corrections enabled by Voyager, the apparent anomaly in Uranus’ orbit was proven to be a math error: there was no Planet X after all.
So what the hell was Pluto?
Eventually, it was determined that Pluto had less than 0.2% of its initially estimated mass, and that its appearance near the predicted position of Planet X’s orbit was just a bizarre coincidence. In spite of this, it retained its provisional planetary status; it had already captured the public’s imagination, and the fact that Pluto was the only “planet” to have been discovered by an American created enormous political pressure against classifying it as anything else.
This would remain the status quo until the discovery of additional outer-Solar-system objects as large or larger than Pluto in the mid 00s - most notably Eris - forced the classification issue to be resolved.
TL/DR version: Pluto was never uncontroversially classified as a planet in the first place. It just happened to coincidentally be near the orbit of a hypothetical ninth planet that was later proven not to exist, and sort of inherited the planetary status of its phantom sibling on a provisional basis due to a combination of institutional inertia and politics.
(As icing on the cake, at the time of this posting, early 2016, there’s new - albeit controversial - evidence that there really is a mysterious ninth planet lurking out there. Note, however, that this conjecture is based on a completely different set of anomalies from the ones that led to the Planet X hypothesis.)
