small つ

[japanesewords]

The chisai tsu creates a harder sound or a pause. This is not something common in the English language and took me a while before I could even hear the difference. It is important though since the a word containing or not containing it can have a very different meaning.

[Nyororin]

My favorite one to hear people screw up on is 簿記/勃起

A HUGE difference in meaning between the two, but oh so common a mistake when foreigners are doing business in Japanese.