When you create a rewrite action, the NetScaler verifies that the expression you used to create the action is safe. Expressions created by the NetScaler from run-time data, such as URLs contained in HTTP requests, can cause unexpected errors. The NetScaler reports expressions that cause such errors as unsafe expressions.
In some cases, the expressions may be safe. For example, the NetScaler cannot validate an expression that contains a URL that does not resolve, even if the URL does not resolve because the Web server is temporarily unavailable. You can manually bypass the Safety Check to allow these expressions.