I forgot a resistor on my PCB prototype - how do I add one in?
If your PCB has a solder mask you can fix this with an SMT resistor even when traces are close together. Cut the trace and scrape the solder mask away on both sides of the cut, over a few mm. Solder an 0603 or 0402 over the cut. If I patch like that I actually make two cuts a mm apart, and remove the trace(*) between both cuts to make sure there's a decent interruption of the trace.
Alternatively you can do the same and solder a PTH resistor, whose wires you first bent so that they lay flat on the trace. Won't probably work as well on narrow traces, like 0.5mm.  

This type of knife allows for very accurate work (pingswept mentioned the brand X-Acto):  

  

Success!



(*) davidcary uses a dental pick for this,   



but the tip of the X-Acto is fine and sharp enough to do it with that.