SN1 reaction
Two common side reactions are elimination reactions and carbocation rearrangement.   If the reaction is performed under warm or hot conditions (which favor an increase in entropy), E1 elimination is likely to predominate, leading to formation of an alkene.  At lower temperatures,  SN1 and E1 reactions are competitive reactions. So, it becomes  difficult to favor one over the other.  Even if the reaction is performed cold, some alkene may be formed.  If an attempt is made to perform an SN1 reaction using a strongly basic nucleophile such as hydroxide or methoxide ion, the alkene will again be formed, this time via an E2 elimination.  This will be especially true if the reaction is heated.  Finally, if the carbocation intermediate can rearrange to a more stable carbocation, it will give a product derived from the more stable carbocation rather than the simple substitution product.