Vertical datums are changing. One of the most common datum issues is converting from NGVD 29 (National Geodetic Vertical Datum of 1929) to NAVD 88 (North American Vertical Datum of 1988). The original FEMA Flood Maps are a good example. The FEMA maps were originally completed NGVD 29. In areas were maps have been updated the vertical datums have been changed to NAVD 88.
The elevation difference between the two datums vary by location. The following is an easy to use conversion tool.