It was a while ago now, but I remember copying the code out to a controlled isolated environment on my local machine, picking the first line and making sure it reproduced.

After that it was a matter of just going through the usual steps of working out why something isn't working. I think in this instance I actually ended up manually retyping the code, and when getting "identical" code that worked that was when the penny dropped and I realised something was not what it seemed. If you paste suspect characters into something like http://unicode-table.com/en/ it will tell you right away what it really is.

Weird bugs can respond well to brutally dumb debugging. Open file and search for ; perhaps? Or grep etc. When you can see semi-colons but your find tool can't, that's a worry.

But yeah getting to that point relies on pure inspiration, in my case.

Personally, my vim status line has an indicator which shows the hex code for the rune currently under the cursor.

After fighting against word processor quotes, it's become second nature to double check it periodically.

Something similar happen to me, but with that fancy quotes. I spotted by doing a "binary-search weird bug hunt". Cut half of the code off and see it it's still complaining, if it's, cut the other half, and so on.

I had a similar problem. Copied a code snippet. Ruby started complaining about an undefined function. After nearly going mad, and then looking at the source through a hex editor, you could see Unicode whitespace. I have yet to forgive ruby or Unicode whitespace. Or the chat utility from whence I copied.