  The recently reported excess of events at HERA compared to QCD calculations
impels us to examine all possible Standard Model explanations before invoking
``new physics''. We explore the possibility of adding an unusual, but small,
component of additional quarks at large x (beyond x>0.75) as a way to increase
the predicted SM cross-section in the HERA kinematic region by the QCD
evolution feed-down effect. We describe various scenarios under which this can
be achieved while maintaining good global fits to all established data sets.
This implies a much larger SM uncertainty than commonly assumed. In addition,
the modified parton distributions provide another possible mechanism to account
for the CDF high-pt jet excess which occurs at similar x and Q^2 values.
