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xkcd #3236: Border Message
(imgs.xkcd.com)
Title text:
Thanks to differences in logging regulations, the messages actually turned out to be visible from the air.
Transcript:
Transcript will show once it’s been added to explainxkcd.com
Source: https://xkcd.com/3236/
Been there and got 6 messages from my carrier telling me I was entering another country