Some things were not supposed to be Turing-complete. This is a collection of such accidents.

Stuff which is somehow limited (stack overflows, arbitrary configuration, etc) is still considered Turing complete, since all "physical" Turing machines are resource limited.

## C++ Templates

Although they were initially not supposed to, C++ templates are Turing-complete. For proof look at this paper (pdf).

## X86 MMU

The page fault handling in X86 can be used to implement a simple machine. Basically, a page fault pushes a word to the stack, which might underflow generating another trap. This mechanism provides a "subtract and branch if less than or equal to zero" instruction. Enough to implement a Turing machine. Demo video Talk video

## Magic: The Gathering

Magic is a card game. Apparently, the rules are complex enough to reach Turing-completeness.

## HTML5 + CSS3

While older HTML/CSS versions are not, with some new additions, you can write a rule 110 automaton. The original page has moved here and there is also Github and a video.

## Minecraft

Maybe this one was intentional, but the complexity of the computers people build in Minecraft is impressive. video example

## SQL

SQL is usually not considered to be Turing-complete. However, with the features Common Table Expressions and Windowing, SQL is Turing Complete. The proof is in these slides. Also, look a Mandelbrot in SQL:2008. A broader exploration of various SQL-turing-machines was done by Fabien Coelho.

## (C Preprocessor)

The C preprocessor is only Turing-complete, if executed in a loop, which feeds the output to the input ad infinitum. An example has won the IOCCC 2001 contest. Look into the herrmann1 entry. Nonetheless included in this list for coolness.

## Apache Rewrite Rules

Apache comes with the mod_rewrite plugin to rewrite URLs. These rules ultimately are a Turing complete string rewrite system. Though the default configuration sets very low limits to recursion.

## (Pokemon Yellow)

A Pokemon game, which is finished in 1minute 36seconds. The interesting point about this speedrun is the bug it exploits. Turns out the game logic itself is Turing-complete in the sense that you can rewrite the assembly itself with game actions. For example, someone turned the game into a MIDI player.

## Scala Type System

Apparently, you can implement the SKI calculus (which is Turing complete) in Scala types. However, the compilers stack overflows at some point.

## MediaWiki Templates

In MediaWiki you can define templates. Since they provide recursion, you can apparently implement lambda calculus.

## Little Big Planet

Yet another game, where you can build a basic computer. For example, Conways Game of Life.

## Server Side Includes

While SSI was design as a scripting language, loops were not planned. The trick was to find a way for recursion. The recursion is limited in all web servers.

## Sendmail

The venerable mail server is known for its arcane configuration. Turns out the configuration is also turing complete.

## Border Gateway Protocol (BGP)

BGP is an internet backbone technology, which manages the routes your data packages take. The paper "Using Routers to Build Logic Circuits: How Powerful is BGP?" prove Turing-Completeness via logic gate comparison.

## Excel

It is no surprise, since Excel includes a scripting language. However, there is a nice blog post, which shows how to encode a Turing machine in Excel using only formulas.