You are right that data is missing. Could it be the IBM went on vacation and shut down the whole company and Citibank celebrated an extended banking holiday?
No, all kidding aside, Yahoo buys those data from an outside supplier and that guy goofed. However, the only place I have found to get the attention of a Yahoo staff member if something in Yahoo Finance does not work is in the category 'Yahoo Products,' subcategory 'Yahoo Finance.' That is the place that is checked by Yahoo staff religiously and you have an almost certain chance to get the attention of and an answer by a their staff. I have done it and gotten a very good response and eventual resolution of the problem, because in this case they did not get what they paid for and they can get that resolved with their vendor.
http://finance.yahoo.com/q/hp?a=00&b=2&c...
Check IBM (IBM): No historical data between Jan 10, 2014 and Jan 21, 2014. Check Citigroup (C): No historical data between Jan 10, 2014 and Jan 27, 2014.
