The \ character is "line continuation" for the pre-processor.
My suspicion is that there was a line-ending problem. The editor was displaying this...
int injon = 0; // Injector on time
int injstate = LOW; // Used to control digitalWrite on injPin
The compiler was seeing this...
int injon = 0; // Injector on time int injstate = LOW; // Used to control digitalWrite on injPin
Why did it work on my system? Probably a difference in operating system, JVM, or internet browser. Any one of those could have silently corrected a line-ending discrepancy.
Why did removing a \ fix the problem? Maybe the editor checked and corrected the line-endings because of a change.
But, that's all just a shot-in-the-not-very-dark-because-it's-a-full-moon-lit-night theory.