The current Protein Data Bank (PDB) contains about 40 000 protein structures with approximately half a million incorrect atom positions resulting from erroneously assigned asparagine (Asn) and glutamine (Gln) rotamers. These errors affect applications in protein structure analysis, modeling and docking and therefore the detection, correction and prevention of such errors is highly desirable. We present NQ-Flipper, a web service based on mean force potentials to automatically detect and correct erroneous Asn and Gln rotamers. The service accepts protein structure files formatted in PDB style or PDB codes. For an Asn/Gln side-chain amide NQ-Flipper computes the total interaction energy with the surrounding atoms as the sum of pairwise atom-atom interaction energies. The energy difference between the original and the alternative rotamers identifies the correct configuration of the amide group. The web service lists the interaction energies of all Asn/Gln residues found in a PDB file and shows the structure and offending residues in an interactive 3D viewer. The corrected protein structure is available for download in various compression formats. The web service is accessible at http://flipper.services.came.sbg.ac.at.