It is just like any other facet of aviation: Any human might make an error at any time. Through hard lessons, we have developed procedures and crosschecks to minimize the probability that a single error will be catastrophic. However, we must follow those procedures and perform those crosschecks to continue the chain.
While the Dispatch Release may not have an accurate final weight, it is unlikely to have a 100 Tonne or 100,000 lb discrepancy. Safety margins will likely render 10-Tonne errors harmless, and procedures such as dialing in a preliminary V2 into the MCP based on the flight plan weight should alert to a large error.