It can also happen when there is CSS like the following:
Code:
p {margin: 0;
text-indent: 1em;}
.center {text-align: center;}
If that center class is applied to any <p> tag (eg: <p class="center">), then the indent of 1em WILL apply to the centred text.
I actually have had plenty of problems when converting mobi files from Amazon with centred text having an indent applied. This usually happens though, because for whatever reason, there is usually an explicit indent applied to the center class. (I don't know if the indent is present in the original mobi, or if it is introduced in the conversion stage).