Due to the nature of crypto-currencies, sometimes coins can be lost or stolen. The Mt. Gox incident is one wide-spread example of this happening. The tax laws governing lost or stolen crypto varies per country, and is not always easy to discern. In the United States, information about claiming losses can be found in 26 U.S. Code § 165 - Losses.
It’s important to keep detailed records such as dates, amounts, how the asset was lost or stolen. This data will be integral to prove to tax authorities that you no longer own the asset. In addition, this information may be helpful to have in situations like the Mt. Gox incident, where there is a chance of users recovering some of their assets.
You can add lost and stolen crypto to the Spending Tab in BitcoinTaxes. A summary of your lost or stolen crypto can be found on the Reports Tab. It's important to add this data as it ensures your crypto balances are accurate.