What is the major side product of the reaction between 2-bromo-2-methylpropane and hydroxide ion?
On addition of hydroxide to t-butyl bromide there are two competing reactions, substitution and elimination. Depending on the conditions one of these reactions will predominate but both will always be present. If you are taking the $S_{N}1$ as the main reaction then the elimination ($E1$) side product will be 2-methylpropene, which is a gas at room temperatures and pressures.