Aside from retaining caffeine longer, is there any bad side effect to consuming lots of coffee while on Lamisil (terbinafine) tablets for fingernail fungus?

no. There are no proven side effects.
Unknown. Lamisil (terbinafine) decreases the clearance of caffeine by about 20%. U need to be careful if u have a condition that can be aggravated by too much caffeine (ex. Hi blood pressure, anxiety, etc...) . Nonetheless, talk to your dermatologist about "pulsing" lamisil (terbinafine). Essentially, it enables you to take half the number of tablets of Lamisil (terbinafine) during the 3-4 month treatment without compromising any efficacy.