  We study the impact of nonrenormalizable operators in flipped SU(5) that can
generate a large mu term, R-parity violation, and rapid proton decay. While our
motivation is to determine whether F-theory can naturally realize flipped
SU(5), this analysis is general and leads to a characterization of symmetries
capable of controlling such operators and should be independent of F-theory. We
then discuss some specific implications for F-theory model building, where a
significant mu problem is unavoidable. Finally, we mention previously noted
difficulties associated to engineering GUT-Higgs fields in F-theory, suggest a
direct engineering of SU(5) \times U(1)_{\chi} as an alternative, and present a
sample construction of this type.
