Telepod was the first invention she made that actually worked (Gato doesn't count). And whether it worked is still debatable. How does she go from town fool to time conquerer? Is it really plausible that a crappy inventor could watch her friend disappear and instantly 1. know he travelled through time and 2. know how to tweak the telepod technology to allow her to do the same?
She went from Wile E. Coyote to Albert Einstein!
the whole "traveled through time" thing wouldn't be hard for her to figure out... shes a geek/nerd, so she HAS to be some kind of history buff. With her knowledge of the past, I'm pretty sure she could easily have put 2 and 2 togethor by the time she reached the castle. That, and she probably could have gone up and asked someone / overheard something related to that time period.
Its not that the telepod was teh first thing that actually worked, its just that so many times her inventions, despite being, usually, quite well planned out and well made, would often have some sort of error or mistake in them... and rather than taking the time to debug BEFORE showing off her intellect, she usually just builds/sets up things on-site. Its kindof like making a video game, and releasing it without ANY debugging, error checking, or beta testing.
^^ btw that was all speculation