Interesting page on Wikipedia about Stop sounds.
Stop sounds or stop consonant sounds are some of the sounds a stutterer can have the most trouble with. The 6 main sounds are p, t, k and b, d, g. Say these sounds each one at a time and feel the airflow stop. When the airflow stops it is normally the time a stutterer will have the most trouble starting again. So these sounds are considered the danger sounds.
http://en.wikipedia.org/wiki/Stop_consonant