• Syntax error: typo, missing newline, incorrect indentation in Python, etc.
• Unknown extension API. Perhaps the script was written against a newer version of REAPER.
• Incorrect use of the Extension API. Verify how the function parameters and return value should be used in the REAPER-generated ReaScript documentation.
• Lua: invalid "objects". Check variables for nil before passing to functions.
• Python: REAPER cannot find the Python library. Is the proper 32/64-bit Python installed? Is it enabled in Preferences?

• SetExtState() and GetExtState() allow ReaScripts to set and get key/value pairs that will persist between ReaScripts, and optionally persist between REAPER instances as well. For example, a Python module called "utility" might want to remember that some flag is set:

  RPR_SetExtState("utility", "someflag", "2", True)
  val=RPR_GetExtState("utility", "someflag") # even if called after REAPER is closed and reopened, val will be equal to "2"
  

• SetProjExtState() and GetProjExtState() allow ReaScripts to save data within the project RPP file.

There are also API functions for manually creating undo points, as well as API functions to Undo and Redo previous points.

The best way to manually create an undo point for your script is to call Undo_BeginBlock() before most of your code, and Undo_EndBlock() after your code finishes.

• These functions operate on the active project only (if multiple tabs are open) -- do not switch project tabs from your code while in an Undo block.
• You must always pair Undo_BeginBlock with Undo_EndBlock. If you call Undo_BeginBlock and then fail to call Undo_EndBlock, an undo block will be left open, which may cause undesired undo behaviors.
• The final parameter of Undo_EndBlock is a bitmask of what you would like to add to the undo state. If you are simply calling other actions (via Main_OnCommand or similar functions), this should be 0. If you modify the state of the project directly, it's safest to set the bitmask to -1. A more specific combination of flags may be an optimization, depending on what your script is doing.
• Always give the "desc" parameter of Undo_EndBlock a valid (non-empty) descriptive string.

reaper.Undo_BeginBlock()
m = reaper.GetMasterTrack(0)
reaper.TrackFX_GetByName(m, "ReaEQ", 1)
reaper.Undo_EndBlock("Add ReaEQ FX to the master track", -1)

function run()
  is_new,name,sec,cmd,rel,res,val = reaper.get_action_context()
  if is_new then
    reaper.ShowConsoleMsg(name .. "\nrel: " .. rel .. "\nres: " .. res .. "\nval = " .. val .. "\n")
  end
  reaper.defer(run)
end

function onexit()
  reaper.ShowConsoleMsg("<-----\n")
end

reaper.defer(run)
reaper.atexit(onexit)