Something i have always debated is at what point does a curveball be classified as a slider. We all know that pitchers throw breaking balls with a huge variety and with enough out there, it can be thus assumed that every slight variation in arm slot, velocity, break and rotation rpm can be catalogued. This surely is true because we see such a myriad of differences, albeit sometimes very slight, from one pitcher to another. Because of this fact, it must also be true that there are pitchers stuck halfway inbetween that ground of it being either a curveball or a slider and yet not be classified as a "slurve" either.
This is where it gets interesting because there isn't really any true facts or conditions that clearly separate the two pitches when they get to that fuzzy middle ground that so many of today's pitchers are now throwing within. They are both thrown with very similar arm actions and follow through. The placement of the fingers can be identical on the seams from one to the other. Even the wrist angle and motion can be identical from one pitcher to another. In today's standards, not even the motion of break can determine whether it be a curve or a slider. I have seen sliders with 2-8 break and also with 12-6 break. Sometimes, a pitcher from one game to another will have different movement (break) on his slider or curveball due to differences in release point variations, slight differences in finger placement on the seams, etc. I watch the Braves a lot and see this effect with their closer Craig Kimbrell quite often. Some nights his slider has a short tight break with almost a 12-6 break and other nights that same pitch will not be as tight and move with 2-8 break. Because he throws it hard (around 90 mph) no one questions if it is a curveball because curveballs are not thrown that hard generally speaking. But, sometimes it's around 87-89 mph while his fastball is touching a 100 mph. The differential percentage is basically exactly the same as a curveball here. Curveballs are generally thrown about 12-15% slower than a pitchers fastball. So, an 87 mph slider compared with a 99 mph fastball fits in that differential. Suppose that Kimbrell only threw a fastabll that topped out at 90 mph. His slider, if we hold to his same differential, would only be 77-78 mph. Because of the slower speed of the pitch it would have greater movement and would probably be called a "curveball" and not a slider anymore. So, in this case why isn't his 90 mph breaking pitch called a curveball? It's thrown the same, has the same break (albeit it being faster) as a curveball. Why? Because the standards we have placed (imaginary at best) on breaking balls places mph limits on pitches as a way of differentiating or cataloguing pitches.
This is just one example but it just shows that the imaginary line between a curveball and a slider is so vague at times that it must be true that sometimes a curveball is/can be classified as, a slider and vice-versa.
Whereas it is true by old school standards that a true curve had a much lower velocity, a larger downward break, etc. and a slider had higher velocity, tighter break with more sideways movement, now days pitchers are overwhelmingly blurring those lines more and more. Nowdays, you see everything between a true curve and a true slider (if there even is such a thing, eh eh) with no clear line or point to separate the differences bewteen the two.