Read the caveats!
I'm confused about Ceuta and Melilla though. I thought they were part of Spain, like French Guiana is part of France, and not overseas territories like Gibraltar. Other featured quizzes seem to think so e.g. "Countries by Bordering Flags #2", where the Spanish flag is given for Morocco. And they aren't on Wikipedia's list of dependent territories.
I'd also like to point out that after you move along the coast from Belgium to the Netherlands, you come back to Belgium again. More than once, I fancy.
Apart from that, I don't think there's anywhere you could say the coast goes back to Belgium - all the various Belgian enclaves in NL are well inland.
Belgium/ Netherlands; if you follow the Westerschelde, you’ll end up in Belgium again, but Westerschelde is de estuary of the river Schelde, so you can’t consider it as part of the coastline.
Overall, a great quiz!
What's going on with the Istanbul area - is that intentional?
The way I looked at it yesterday I thought it was overlapping, but it's just the Sea of Marmara almost closed off at either end.
Another thing one can do is trace the entire coast, but it would be very lengthy, and what I did would be more efficient. My process might seem hard at once, but after doing it with 5-6 countries, it will become pretty easy.
For me, it took me around 6-7 hours, over the span of 3 days, to make this SVG, but the first couple hours were just me trying to figure out how to do this by trial and error. If I knew from the beginning how to do this, it would have taken me about half the time.