The code I'm writing here is JavaScript, and by default all strings are considered Unicode. So, to get (say) a "trademark" symbol, I can write String.fromCharCode(0x2122), or write XML markup saying "™" and get a character with numeric value way up in the thousands.

Weirdly, these very-high characters passed right through the pipe - in one end, out the other - in both directions, without breaking. Yet more mundane symbols such as the pound sign ("£") and Latin-1 accented characters would detonate along the way. (The actual symptom was reporting that my SQL Server was stopped, but that was a red herring).

So, that pound sign was character 163 ("%A3" after URL-escaping), but the URL wanted UTF-8, where the pound becomes two bytes: 194 163, or %C2%A3. UTF8 is wonderful and slightly strange - don't ever try working backwards through a UTF8 string!