TServo2049 said:

He said something like, "The best we could do was put some vaseline on the lens and kind of fudge out the wheels." Even in '97 I was confused, because vaseline is clear and wouldn't make an orange blob like that. Then, at some point I also realized that a "vaseline on the lens" trick wouldn't work with a tracking shot.

Yeah, I've always been confused about that as well.

Though it does kind of look like there might be a little bit of smearing over that R2 unit as it goes past. And maybe that guy's legs.

Maybe they tried a little bit of Vaseline first, and it wasn't working out, so they had to take additional measures?

I was reminded of the trick the other day, watching The Dunwich Horror. There's a few scenes of a dream sequence where it looks like they put a fish bowl over the camera with Vaseline smeared all over it (you can even see the brush marks on it). Could've been anything though, but still pretty funny.