Note that this problem is slightly hard to explain here, because this instance currently suffers this bug, but it may not suffer the bug in the future.
I will use "& a m p ;" to represent the ampersand escape sequence, but in reality I mean the sequence without spaces.

If I type an ampersand ('&') character in JIRA in a field with Wiki Style Renderer, then this is escaped out correctly in the HTML that is created, and the user will see "&".
However, if I type "& a m p ;", then no escaping is done and the user will only see "&", however they should see all four characters.

Note that the Default Style Renderer does actually work as expected.