The shapefile has been corrupted. Normally this is due to manipulation of the .dbf file by some dBase-III+ aware utility which is NOT shapefile aware. But in this case, it's probable that you've simply exceeded the valid feature count on the shapefile format (the limit is based on vertex count [~76M vertices], so the actual feature limit is dependent on average vertices per feature; once the .shp writer exceeds 2^31 bytes, it stops trying).
Your best bet to avoid this is to use file geodatabase, not shapefile (FGDB is also much smaller, since it uses compression and variable width strings).
- V
