What happens if someone's bicuspid valve does not close properly?

Aortic insufficiency. When an aortic valve (bicuspid or a normal tricuspid valve) does not close properly, insufficiency or regurgitation ("leaking") results. When mild, this does not have clinical effects, but when severe it can compromise cardiac function.
Aortic regurgitation. If the bicuspid valve does not close properly, one can develop aortic regurgitation which in time may lead to left ventricular enlargement and heart failure. Depending on the degree of the regurgitation ('leakage'), one can treat it with medications and at some point a valve replacement may be needed. You need to have this followed with echocardiograms at regular intervals to monitor progression.