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We obtained new ROSAT HRI and optical observations in the field of theEinstein X-ray source E 0336-248, which we use to identify it as a 19thmagnitude BL Lacertae object at z=0.251 with an X-ray luminosity of 10^45erg/s. It is also a 14 mJy radio source at 20 cm. An emission-line galaxy atz=0.043 that was previously considered a Seyfert identification for E 0336-248is shown instead to be an unrelated, non-active H II region galaxy that lies 78arcseconds from the X-ray source. The resolution of this historical case ofmistaken identity illustrates that discoveries of non-AGN emission-linegalaxies with high X-ray luminosity should be tested carefully. The propertiesof E 0336-248 are similar to those of other X-ray selected BL Lacs, includingits location in an apparent group or cluster of galaxies. Somewhat unusual isthe weak contribution of nonstellar optical light relative to the starlight inthe spectrum of its host galaxy, which raises once again the possibility thateven high-luminosity BL Lac objects may be difficult to identify in X-rayselected samples. We discuss a possible manifestation of this problem thatappeared in the recent literature.Comment: To appear in the Astronomical Journal. 8 pages including figures  (uses psfig.tex, also included

E 0336-248: A New BL LAC Object Found By an Old Einstein