I find it mildly irritating when speakers at conferences or seminar series say something like, "I am not going to explain all this background material or my earlier results because last year I talked about that."
There are several reasons I don't like this.
1. It assumes that everyone present was there last time.
2. It assumes that those present on the last occasion understood and remember the relevant material! [To me that is fantasy.]
3. It seems to be shutting off certain topics for questions and discussion.
Obviously, one can't cover everything and must leave out material. Everyone knows and accepts that. But, I think it is better not to deal with the problem in this way.