I have seen couple of syllabi that used a disclaimer similar to "Instructor reserves the right to change the syllabus at any time". For the instructor side, it could be good, but from students side it could be good or bad, depending on the change. Any other thoughts?
Unless there is an unusually rigid set of controls and policies in the instructor's department, it is generally always the case that the instructor can change the syllabus on the fly. With any decent instructor, there is usually a good reason for doing so, such as incorporating an improvement the instructor has figured out partway through or adding extra emphasis on a section that people seem to be having trouble with. With a poor instructor, on the other hand, I don't think that the situation can be rescued by making the syllabus more rigid.
As such, my interpretation of the disclaimer is that it's simply a way to insulate the professor from complaints of students who take a rigid interpretation. From the students' side, I think it means little, only acknowledging a reality of instruction.
(Somewhat in reference to the other answer and pursuant comments...) Even though I do privately think that my job is to give the students the best education I can, it seems both "fair" and "anxiety-reducing" to make quasi-contractual promises in advance... all the while realizing that the students make no such promises to me, etc. But it's a very asymmetrical situation.
So basically what I try to promise is to assure the anxious, primarily about the grading system, gradelines, and how much material will be "assessed-upon". In effect, such an approach does give openings to those seeking to "game" the system, but I reject a role of "policeman" for one thing, and, for another, see it as undesirable that the earnest (often anxious) students get put-upon for the sake of trying (which is often unsuccessful anyway) to keep at bay the game-the-system people.
The aspect of the syllabus that specifies the extent of the content of the course can be made fairly independent of the grading scheme specification, I think, even if this amounts to declaring some "extra topics" outside the bounds of "what will be tested-on". (In upper-division undergrad courses I'd regularly include will-not-be-on-the-test information, with a few catty remarks about people who only care "what's on the final"... and certainly in graduate courses only talking about what's reasonably testable does a great dis-service to the students, since they'll have a "textbook" picture of something (with the pursuant testability constraints, non-interaction with other topics/ideas, and so on), rather than the "live" version.)
I really do think the "promises about grading system" and "outline of what will be covered (and tested upon?)" are two very distinct things. The grading system should be a bit contractual, I think, given the power discrepancy, and potential for harm/anxiety. The content promises are rather different, since there's not necessarily any adversarial relationship.
At my school faculty could make any changes to the syllabus that they wanted to make, but a copy had to be given to the students in writing (email was okay).
As far as the contract is concerned, it protects the instructor to think of it in that way. If it comes to a grade grievance, the only thing in question is whether the instructor graded the student according to the terms of the syllabus; everything else is extraneous. It seldom happens that a grade grievance makes it all the way to a committee, but if it does, this protects the faculty member.
As a teaching assistant in grad school, I once strayed from a professor's syllabus and was reprimanded by said professor, who said that the syllabus has contractual weight and serious problems can result from sudden deviations from it.
I never investigated the true legal weight that was claimed by that professor, but even if its legal weight is very small, it remains a fact that American culture has grown increasingly litigious. It is common practice to put in disclaimers to this effect "just in case".
In law there are clear violations, clear non-violations, and a frequently vast grey area in between them. Most universities, corporations, etc. make it a point to avoid the grey area completely. You just don't want to risk someone taking you to court over a possible violation.
Similarly, as an instructor it's a major pain if one or more of your students decide to go to your chair or dean to complain about violations of your syllabus. Being able to point at the syllabus and say "nope, it specifically says this could/would happen" will fend off the more spurious claims.
When changes are made they are almost always minor, and are announced in advance or plainly necessary. Usually it's a matter of changing the planned lectures due to falling behind (or even getting ahead); adjusting to the strengths and weaknesses of the particular class; removing optional topics in favor of something else; or giving a class-wide re-examination when the class as a whole performs unacceptably poorly. You still have to stay within the broad scope of the course description: a calculus course has to stay about calculus, a course about 18th century China can't turn into a course on modern Japan, etc.
The syllabus is not just a contract with the students, but more so a contract with whoever will have to teach the classes after yours. If you just leave something out, or cover it lightly to make space for "more interesting" subjects, you are doing your students (and those who will have to clean up afterwards) a disservice.
Here you can request the syllabi for the classes you have taken, in case you want to e.g. apply for continuing study elsewhere (and also for exchange students visiting from abroad). So it is important to have them quite clear in what subjects are covered, and adhere to that. Grading is more a local matter, but shouldn't be changed without the explicit consent of the affected students.