Since the start of September approximately keyboard shortcuts in OS X have been acting strangely.
Many of the common shortcuts (cmd+z, cmd+v, cm+c, cmd+w etc) select the previous word rather than eliciting their usual action. Further presses simply continue the backward selection of words as if I were pressing shift+option+left arrow.
This does not occur immediately. For example, in Apple's Mail programme, upon loading I can press cmd+n and a new window in which to compose an email appears. However, after commencing typing, any further use of the keyboard shortcuts elicits the strange behaviour.
The behaviour also occurs in Safari once some text has been highlighted. As with mail, the standard shortcuts operate as, well, standard before some text has been highlighted. Once it has been highlighted however, all shortcuts assume the "select word backwards" functionality.
The problem does not manifest itself in Pages or Scrivener. However it does occur in Apple's Notes programme.
I am using a 2012 mac mini, currently with Mavericks. The problem first appeared while running Mountain Lion.
I had some keyboard modification programs running, e.g. keyremapper and an apple script to select a current word via Fastscripts. Suspecting the latter might be the cause of my woes, since the problem roughly coincided with its first use, I have removed it and all the rest of them to boot. I then reset the keyboard to its defaults and restarted the system.
I have tested it against Apple's wireless bluetooth keyboard and a logitech usb receiver cordless keyboard while web searches have failed to return even a ball park level of relevant information.