I'm using one of my plugins to render a page from a controller by calling RenderTemplate and this gets dumped out at the top of my page:

Note the mention of Slimsy in the stack trace. Here's the text version of the stack trace (for the search engines):

[HttpException (0x80004005): Filtering is not allowed.]
   System.Web.HttpResponse.set_Filter(Stream value) +3470552
   Imazen.SlimResponse.SlimResponseModule.RegisterMarkupFilter(Object sender, EventArgs e) +262
   System.Web.SendResponseExecutionStep.System.Web.HttpApplication.IExecutionStep.Execute() +142
   System.Web.HttpApplication.ExecuteStep(IExecutionStep step, Boolean& completedSynchronously) +69

This is the code I'm using to render the page: https://github.com/rhythmagency/render-page-to-file/blob/master/RenderPageToFile.App/Controllers/RenderController.cs#L61

Not quite sure what's going on here, but it seems to be related to Slimsy.