Astrometric and photometric corrections for the 34th row error in HST's WFPC2 camera

A small manufacturing defect, known but rarely noted, occurs at approximately every 34th row of each of the chips of WFPC2. This defect induces errors in photometry at the 0.01-0.02 mag level for about 6% of all stars, and periodic errors in astrometry for all stars, with an amplitude of 0.03 pixel. We derive simple corrections for these errors. The errors exist in all astrometry and photometry previously done with WFPC2, but they can be avoided in future work by applying the corrections that we give here.