Something that has hit me more times than is ideal is opening times. A surprising number of travel guides do odd things like say 'open Tuesday to Sunday, closed 1st Sunday of the month.' At a quick skim its easy to miss the 2 key things, closed some Sundays, closed all Mondays. And unhelpfully the format of the information is often inconsistent within the same publication. Invariably the info is out of date anyway, and prices are considerably higher than the guides states. We usually used Lonely Planet or Rough Guide, but the lack of up to date opening times was a continual pain (out of date prices is annoying, but you can spend more and get in, but if the place is closed, you're stuck).