w, h = 100, 100
img = {getInt = function (x,y)
  return x > 2 and x < 98 and y > 2 and y < 98 and -1 or
0
end}

ofs = 0

while ofs <= w/2 and ofs <= h/2 do
  colors = {}
  
  for y=ofs, h-ofs-1 do
    colors[img.getInt(ofs, y)] = 1
    colors[img.getInt(w-ofs-1, y)] = 1
  end
  
  for x=ofs, w-ofs-1 do
    colors[img.getInt(x, ofs)] = 1
    colors[img.getInt(x, h-ofs-1)] = 1
  end
  
  list, n = {}, 0
  for color, _ in pairs(colors) do
    n=n+1
    list[n] = color
  end
  
  if #list > 1 then
    return ofs == 0 and "no border found" or "border thickness: "..ofs
  elseif lastcolor == nil then
    lastcolor = list[1]
  elseif lastcolor ~= list[1] then
    return ofs == 0 and "no border found" or "border thickness: "..ofs
  end
  
  ofs = ofs+1
end

Bug description: Click "test run". Sometimes it returns "ndrder..." instead of "border...".

Began life as a copy of #304