In front of the match is a non-keyword character, or this is where the line or insertion starts. Or type CTRL-V before the character after the abbreviation. Type CTRL-V twice somewhere in the abbreviation to avoid it to be replaced. A CTRL-V in front of a normal character is mostly ignored otherwise. There are no default abbreviations.
Abbreviations are never recursive.
But abbreviations can be mapped. Abbreviations are disabled if the 'paste' option is on. Remove all abbreviations. It is possible to use special characters in the rhs of an abbreviation. CTRL-V has to be used to avoid the special meaning of most non printable characters.