Starting with frame 256, the xkcd servers began to serve two different sets of image URLs to different users. This lasted for three newpix.
After this problem was resolved, one set of images was overwritten with the content of the other set, so that people who follow the comic through the xkcd image links, will get a consistent (canonical) set of images, no matter which image URL they use.
If you have downloaded the frames before this was fixed, though, as this site has done, you can still see the original, different frames. These frames have not been re-used, and are most likely not present on the xkcd image servers.
Thus, the original copies of these frames are apocryphal.
Some time after frame 2440, five extra frames appeared in short succession which showed a meteor shooting through the sky.
These frames are not counted in the regular sequence, as they appeared so much more rapidly than other frames, and did not change the stars in the background. Also, the (Inside) time between frame 2440 and 2441 (the frame after the meteor sequence) appears to be the same as between any other two frames in this scene.
Yes, in two different ways.
If you want to link directly to an image, e.g. frame 123, use
This works with all frame numbers as listed on this site. For all numbers except for "1", this redirects to the image on the xkcd servers, for "1" it redirects to the downloaded copy (since the original xkcd URL is unknown).
If you want to link to a page containing the frame, use
This works for the canonical frame numbers on this site, as well as Geekwagon (prefixed with "gw") and Aubron frame numbers (prefixed with "ba"), as well as "hashes", local file names, and even UTC date/time stamps in the form "YYYYMMDD_HHMM". (In the latter case, the last frame no later than that date/time will be shown.)
The following links all point to the same frame:
If you want to link to a random frame, you can use http://xkcd.mscha.org/frame/random to link to the image itself, or http://xkcd.mscha.org/viewer/random to link to the viewer.