遇到一个奇怪问题，在有时更新ASP.NET程序之后或者某种特定的情况下，会出现这样的错误：
Execution permission cannot be acquired.
Description: An unhandled exception occurred during the execution of the current web request.
Please review the stack trace for more information about the error and where it originated in the code.
Exception Details: System.Security.Policy.PolicyException: Execution permission cannot be acquired.
Description: An unhandled exception occurred during the execution of the current web request.
Please review the stack trace for more information about the error and where it originated in the code.
Exception Details: System.Security.Policy.PolicyException: Execution permission cannot be acquired.
重启IIS就会恢复正常。
经过分析，发现问题与下列场景有关：
应用程序池以ApplicationPoolIdentity用户身份运行，Users用户组对bin文件夹没有修改和写权限。