The properties of organic-semiconductor/insulator (O/I) interfaces are critically important to the operation of organic thin-film transistors (OTFTs) currently being developed for printed flexible electronics. Here we report striking observations of structural defects and correlated electrostatic-potential variations at the interface between the benchmark organic semiconductor pentacene and a common insulator, silicon dioxide. Using an unconventional mode of lateral force microscopy, we generate high-contrast images of the grain-boundary (GB) network in the first pentacene monolayer. Concurrent imaging by Kelvin probe force microscopy reveals localized surface-potential wells at the GBs, indicating that GBs will serve as charge-carrier (hole) traps. Scanning probe microscopy and chemical etching also demonstrate that slightly thicker pentacene films have domains with high line-dislocation densities. These domains produce significant changes in surface potential across the film. The correlation of structural and electrostatic complexity at O/I interfaces has important implications for understanding electrical transport in OTFTs and for defining strategies to improve device performance.