The UK experimented with electronic tabulation of optical scan ballots and they ran into numerous problems in a couple of places. As Computerworld reports:

The results of Scotland’s elections have been thrown into chaos and results delayed by technical problems with the newly introduced electronic counting system.

Scotland went to the polls yesterday for both the local government elections – using a transferable vote system, with voters allocating numbers to their preferred candidates – and the Scottish Parliament elections, where the traditional “first past the post” system is used and voters mark the ballot with an X.

DRS provided specialised technology designed to read and recognise both X marks and the hand printed characters used on the transferable voting ballot papers. But a spokesperson for the Scotland Office said: “Counts have been delayed quite significantly today. They have not been declared when we thought they would be.”