Went to get smog test. Speedometer messed up after?

[2g00d4u]

So after getting a smog test done and driving back home, I noticed right when I got home the speedometer got all faulty. Im at park/idle and it's reading 15mph. Went on the freeway and wouldnt go over 45.

I slap the dash a few times, and whenever I do, the meter drops down a bit.

WTH could the guys at the smog station do to mess this up?

The car is a stock 90 s13 auto.

I wiggled the wires around (from the searching I found, Im guessing it's where the starter is located at. there's like 3 different wires hanging down there) to see if it changes anything, but nope.

Did the sensor just happen to die out of nowhere?