When Sheldon writes equations just before Kripke walks in, he draws a green arrow in fading ink. The arrow becomes more visible and changes shape between shots.
When Howard and Raj receive their action figures, the NFPA 704 symbol on the lab door is oriented incorrectly. The red diamond (flammability) is supposed to be on top, with blue (toxicity) to the left, yellow (reactivity) to the right, and white (other warnings) on the bottom.
When the action figures arrive, the sign on the door is misspelled, it says 'resticted area'.
The first object Howard and Raj 3D-print is a pea-whistle (or referee's whistle), which contains a freely moving ball. This whistle including a moving part can not be 3D-printed in one go but would require 3 different parts to be printed separately (each half of the whistle and the ball) and then assembled.
Sheldon is a theoretical physicist. The project he and Kripke are working on would be in the field of Engineering or applied physics.