When the woman at the bar brings Jack the mustard which she forgot, Jack puts his napkin in front of his mouth. A fraction of a second before that, you can see the down-side of his face which should be invisible. Instead of nothingness you see it's all black. That is material used for the special effects, but they didn't 'make it invisible'.
A purely technical error with all "invisible man/woman/boy" movies is that the eye works by interacting with light incident upon the retina. The Invisible Man is made so by being rendered transparent to visible light. This means that his eyes no longer interact with light - so he ought to be completely blind. (However, a healthy suspension of disbelief makes for a great story.).