I found myself scratching my head for a while over this while modifying an LSL script to run on 3RG. The error message was:
Error CS1009: Unrecognised Escape Sequence
It turns out that it’s an C# compiler error caused by an “unexpected character followed by a backslash in a string”. In my case I had a message to llSay() and I’d used “\n” to create a new line and then inadvertently put an extra backslash, something like this:
llSay(0, "Some message \n\Some message on a new line");
In SL, the script just compiles, but on 3rd Rock Grid (and presumably many other Open Simulator grids) it fails with that error and the editor claims it’s on a line way , way above where the error actually occurs.