My mother once asked me to bring her a pin, and instead I brought her a pen. Are tones more problematic than homophones (and accent-induced homophones) ?
má http://mandarin.about.com/library/audio/tones/2.mp3
vs.
"Bring me the mǎ"
mǎ http://mandarin.about.com/library/audio/tones/3.mp3
One will get you some hemp, the other a horse.