If you have a version older than 7.4.775, adding longest to completeopt breaks autocompletion, because it automatically inserts a whole entry rather than inserting the longest common prefix. To reproduce, create a buffer with this content:
present
prepare
then :set completeopt+=longest, :MUcompleteAutoOn and, in Insert mode, type pr. You should see that prepare is inserted in the buffer. If you turn off MUcomplete and type pr<c-x><c-n> instead, only pre is inserted.
[The following issue was reported by @Felixoid].
If you have a version older than 7.4.775, adding
longesttocompleteoptbreaks autocompletion, because it automatically inserts a whole entry rather than inserting the longest common prefix. To reproduce, create a buffer with this content:then
:set completeopt+=longest,:MUcompleteAutoOnand, in Insert mode, typepr. You should see thatprepareis inserted in the buffer. If you turn off MUcomplete and typepr<c-x><c-n>instead, onlypreis inserted.