Generic Turing machine programs are unreasonable: [the computation head will fall off the beginning of the tape](http://jdh.hamkins.org/haltingproblemdecidable/). 

Basically, the situation is that on the usual one-way infinite tape model, a random program causes a random-walk behavior for the head position, and consequently by Polya's recurrence theorem, it follows that almost all Turing machine programs lead eventually to the situation where the head attempts to move left from the left-most cell, causing the head to fall off the beginning of the tape. Indeed, almost all programs do this before repeating a state. This is not difficult to see, if you think about what a random program line in a huge program will do: write something random, randomly go left or right, and pick a random new state. If the states have not yet repeated, then exactly half of the programs go left and half go right from whatever configuration you're at.