Middleware that trolls front end devs. Use in conjunction with LimitsMiddleware and FakeReverseProxyHost to emulate real-world scenarios. Mwhahahahah!

ChaosMiddleware.cs

namespace Chaos
{
    using System;
    using System.Collections.Generic;
    using System.Threading.Tasks;
    using Microsoft.Owin;
    using AppFunc = System.Func<System.Collections.Generic.IDictionary<string, object>, System.Threading.Tasks.Task>;
    using MidFunc = System.Func<
        System.Func<System.Collections.Generic.IDictionary<string, object>, System.Threading.Tasks.Task>,
        System.Func<System.Collections.Generic.IDictionary<string, object>, System.Threading.Tasks.Task>>;

    public static class ChaosMiddleware
    {
        private static readonly List<AppFunc> s_monkeys;

        static ChaosMiddleware()
        {
            s_monkeys = new List<AppFunc>
            {
                // Bunch of status codes
                env => WriteStatus(env, 400, "Bad Request"),
                env => WriteStatus(env, 403, "Forbidden"),
                env => WriteStatus(env, 404, "Not found"),
                env => WriteStatus(env, 408, "Request Timeout"),
                env => WriteStatus(env, 429, "Too many requests"),
                env => WriteStatus(env, 500, "Internal Server Error"),
                env => WriteStatus(env, 502, "Bad Gateway"),
                env => WriteStatus(env, 503, "Service Unavailable"),
                env => WriteStatus(env, 504, "Gateway Timeout"),

                //TODO
                // - Malformed Content-MD5 request & response
                // - drop properties from Json
                // - corrupt encoding
                // - change header name casing
            };
        }

        public static MidFunc Create(int errorRatePercent, string startWithSegment)
        {
            var segment = new PathString(startWithSegment);
            return next =>
            {
                var random = new Random();
                return env =>
                {
                    var context = new OwinContext(env);
                    if (!context.Request.Path.StartsWithSegments(segment) || errorRatePercent == 0)
                    {
                        return next(env);
                    }
                    var i = random.Next(1, 100);
                    if(i >= errorRatePercent)
                    {
                        return next(env);
                    }

                    i = random.Next(0, s_monkeys.Count - 1);
                    return s_monkeys[i](env);
                };
            };
        }

        private static async Task WriteStatus(IDictionary<string, object> env, int statusCode, string reasonPhrase)
        {
            var context = new OwinContext(env);
            lock(Program.ConsoleWriterLock)
            {
                var foreground = Console.ForegroundColor;
                Console.ForegroundColor = ConsoleColor.White;
                Console.WriteLine(@"CHAOS MONKEY STRIKES \@(o･___･o)@/ {0} {1}", statusCode, context.Request.Uri.PathAndQuery);
                Console.ForegroundColor = foreground;
            }
            context.Response.StatusCode = statusCode;
            context.Response.ReasonPhrase = reasonPhrase;
            context.Response.ContentType = "text/plain";
            await context.Response.WriteAsync(reasonPhrase);
        }
    }
}

Your monkey could be better; may I propose @(◔ ◡ ◔)@