AVR ISP - SCK pin shared with LED
Well, there are some rather theoretical problems you should be aware of.


Adding a DC load onto your SCK line will reduce the maximum voltage that the chip will see during programming. This is because you can never be sure on internal resistance of the programmer's outputs. It may have a 100-470 ish Ohm series resistor to prevent any sort of damage. That means driving a load with pretty much the same resistance will kill half of your waveform.
Diodes tend to have certain capacitance across themselves. This mainly causes problems during the switching transition (from reverse to forward bias, if I remember correctly). So, you may find your SCK signal doing all sorts of ringing and what not.


But anyway, I would be more concerned about the former. You may get a new programmer with higher series resistance and it will stop working.

So, if you really have to pins to spare, get a transistor and drive the LED with it. I'll provide more info if needed.

Cheers.