Boingboing's link to "Learning Mandarin is hard for native English speakers" rings true for Cantonese as well. A little inflexion and wrong intonation can cause a smile or a stern look... or just amusing laughter. I am probably very lucky to be able to tell the difference, but for folks learning to speak the language, it's all down to practice. :)
Even saying some expression depending on the tone can give the wrong impressions. All from experience for all you newbies I suppose.