Use HAProxy with Lua to enforce dynamically generated passwords for basic authentication.
Use simple HAProxy configuration hacks to serve a single static file.
Merge multi-process HAProxy statistics using GNU Awk. This is a duct tape solution, as you should definitely use Lua.
Merge multi-process HAProxy statistics using Lua.
Use HAProxy stats socket to determine current application status.
There are use cases where you need to know which backend server is used, so use specific header to send target server name to backend.
Handle domains that are not defined in the configuration using dedicated backend.
Define backup backend in HAProxy configuration to choose used backend depending on the number of usable servers.
Recently, I have parsed logs of several applications to generate custom weekly reports. It was a fascinating exercise.
Dynamically choose HAProxy backend depending on the HTTP host header, Lua programming language and environment variable.