A three dimensional printing system for manufacturing a three dimensional article includes a movement mechanism, a support tray, a resin vessel, a light engine, a sensor, and a controller. The support tray is mounted to the movement mechanism and has a lower surface for supporting the three dimensional article. The resin vessel includes a transparent sheet defining a lower bound for resin contained therein. The light engine projects pixelated light through the transparent sheet and to a build plane. The controller is configured to (a) receive a start indication for a build process, (b) operate the sensor, (c) determine if polymerized build material is in a flag region from the sensor signal, and (d) if polymerized build material is determined to be in the flag region, halt the build process.