Detect when pages use X-Frame-Options to forbid framing, and offer a link instead
Some pages, notably fanfiction.net, forbid framing using X-Frame-Options. Comic Rocket's crawler should notice this when crawling, and offer a slightly different viewer, which offers the usual navigation but replaces the iframe with a link to the page; I'd suggest making that page open in a new window by default, to keep the navigation around.
For extra credit, it might be possible to pop up the navigation and page in separate windows and have one navigate the other, though I'm not sure if that's actually possible.
The issue seems to be becoming more common; I am recently seeing it with an increasing number of pages.
To add insult to injury, Chrome doesn't display a user-visible error message in this case, which is likely to make average users consider Comic-Rocket broken. Internet explorer by comparison displays a clear error-message instead of the iframe contents.