text: One of the most common sources of errors in ontologies is to have the wrong datatype for data 
  property values. The sooner you catch these errors, the easier they are to debug so it is a good 
  idea to run the reasoner frequently after you enter any values. Note that in some versions of 
  Protégé 5.5. there is a minor bug where the UI may lock up due to an inconsistent data value 
  (e.g., a string value in a property typed for integer). If this happens the best thing to do is save 
  your work if possible, quit Protégé, and then restart it. When you restart it fix the datatype 
  errors*before*you run the reasoner and then run the reasoner to make sure you actually have 
  fixed the error. 