By the time I got the flag small enough to use as an avatar, the quality was too poor. Thanks for the replies! Have a good holiday, but PLEASE don't drink & drive, even if you "just had one drink".
Meaning that something in the code for that page wanted the length of something and it didn't get it. It could be the length of the file, it could be the length of some text generated by the page itself ... only the person who wrote the page can find out for sure - maybe.
As a user I don't worry about such things, I just report them to the webmaster. As a webmaster I try to trap (give the user a meaningful message) all possible errors. I know someone is going to generate one by doing something I haven't thought of, but I try not to lose any sleep over that.