I keep having a problem overlaying shape files that are in California State Plane Zone III (nad 1983) and files that are in UTM Zone 10N (nad 1927) in ArcMap. I sometimes use Global Mapper for translating data, and the two projections line up perfectly in Global Mapper. When I bring the same layers into ArcMap, there are offset by about 280 feet (about the same as the difference between the nad 1927 and nad 1983 projections in this area.) I formerly used ArcMap 9.0, and upgraded to 10.2 figuring it was a glitch in 9.0. I'm having the same problem with 10.2. I don't think its a problem with the data, since everything works perfectly in global mapper. The utm file is a stock usgs 7.5' topo (Felton Quadrangle.) The state plane data includes a variety of gis files from santa cruz county and from the Cal. coastal commmission, among others. It seems like the on-the-fly projection algorithm in ArcMap is buggy. Any ideas?