The Arduino IDE used to warn you when your sketch used too much memory. This warning now seems to have been removed. I compiled a sketch, and was surprised to see it use 175% of data memory...but not give me any explicit warning that this would be unstable, much less be physically unable to load onto the Arduino.
What are program and data memory usage levels that you should be concerned about? I seem to remember the best practice was to keep data memory under 80%. Is this still true? Is there any general rule for program memory?