There has been a glitch in the data processing process at NASA and the word was sent out that they may be revising the May data. Stay tuned.
Please note May 2014 GISTEMP LOTI numbers preliminary due to a glitch with Chinese CLIMAT data. Update to follow pic.twitter.com/1hRF4iJA55
— NASA GISS (@NASAGISS) June 17, 2014
Thanks Chris Mooney for pointing this out.
For the period of the instrumental record, going back to 1881, May 2014 was the hottest May on record. See this post for more details.