Alongside the aforementioned Rhino foreshadowing in The Amazing Spider-Man, there's also a sly reference to the main villain of The Amazing Spider-Man 2, Jamie Foxx's Electro. During the film's mid-credits scene, we see Curt Connors incarcerated in a prison cell of some kind, and following a flash of lightning, the so-called Man in the Shadows (later named as Gustav Fiers in The Amazing Spider-Man 2) appears before Curt to ask him about Peter Parker, and with another flash of lightning, promptly disappears. While it's entirely plausible that the flash of lightning was simply a device to make the scene seem more dramatic, isn't that rather cliched and old-hat by now? Given the enormous attention-to-detail in these movies, is it not more likely that the lightning was foreshadowing the arrival of Electro at the start of the next movie? It certainly fits.