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During an intensive Hubble Space Telescope (HST) Cosmic Origins Spectrograph(COS) UV monitoring campaign of the Seyfert~1 galaxy NGC 5548 performed from2014 February to July, the normally highly correlated far-UV continuum andbroad emission-line variations decorrelated for ~60 to 70 days, starting ~75days after the first HST/COS observation. Following this anomalous state, theflux and variability of the broad emission lines returned to a more normalstate. This transient behavior, characterised by significant deficits in fluxand equivalent width of the strong broad UV emission lines, is the first of itskind to be unambiguously identified in an active galactic nucleus reverberationmapping campaign. The largest corresponding emission-line flux deficitsoccurred for the high-ionization collisionally excited lines, C IV and Si IV(+OIV]), and also He II(+O III]), while the anomaly in Ly-alpha was substantiallysmaller. This pattern of behavior indicates a depletion in the flux of photonswith E_{\rm ph} > 54 eV, relative to those near 13.6 eV. We suggest twoplausible mechanisms for the observed behavior: (i) temporary obscuration ofthe ionizing continuum incident upon BLR clouds by a moving veil of materiallying between the inner accretion disk and inner BLR, perhaps resulting from anepisodic ejection of material from the disk, or (ii) a temporary change in theintrinsic ionizing continuum spectral energy distribution resulting in adeficit of ionizing photons with energies > 54 eV, possibly due to a transientrestructuring of the Comptonizing atmosphere above the disk. Current evidenceappears to favor the latter explanation.

Space Telescope and Optical Reverberation Mapping Project. IV. Anomalous behavior of the broad ultraviolet emission lines in NGC 5548