Some months ago, I encountered a weird problem where a ImageMagick/RMagick application was producing black images on my MacBook Pro. The same code was running mighty fine in production and all other servers. 

require 'RMagick'

# Load a "Base image"
canvas = Magick::ImageList.new.from_blob(open("logo.gif") {|f| f.read})

# Layer another image over the base image - by making the 'background' 
# of this layer transparent, the base image will appear behind the text
# caption nicely
canvas << Magick::Image.read("caption:Text over transparent background!") { 
  self.size = "#{canvas.first.columns}";
  self.pointsize = 60
  self.background_color = '#000000FF'
 }.first

# Flatten everything, and export as a GIF 
# (GIMP / Photoshop users will be very familiar with what this step is about)
canvas.flatten_images.write("logo-output.gif")

Since ImageMagick/RMagick has (to me) a hairy reputation regarding installation, version compatibility, etc, I'd chucked the issue aside as yet-another-install-issue to be solved next time.

Well, today is that next time. A new server was producing black images as well. Sukanta guessed it could be a problem with 64bit OS (oh.. Doh) and suggested I write the tiniest Ruby script to replicate the problem and he'll debug from there.

By the time that tiniest script was written, the bug had revealed itself. The culprit was setting background_color to #000000FF - which the documentation had helpfully noted

Hint:  You can specify the transparent color as "none", "transparent", or "#000000ff" link ยป

Guess not! Only "none" and "transparent" would continue to work.