I'm writing a referee report on a paper. This paper is interesting and, if correct, would be worth publishing. However, I believe they have a major mistake in their method which invalidates their results more or less entirely. I am 90%+ confident that this method is in error, which will affect their results in (possibly unpredictable) ways.
How should I address this in the rest of the paper? I could write the rest of the referee report assuming that they haven't made this mistake - but I think that could be a waste of my time (and their time, since it'll take me longer to do).
If a referee had this sort of problem with your paper, what would you like to happen?
Edited to add: One of the major concerns I have with just rejecting, focusing on this point (as recommended by Raghu and others), is the (time and $) cost of re-running things. Saying, "This is wrong" but not going through the rest of the paper could lead to a second referee report where they've fixed the error and re-done everything. If I then say, "Also, Experiment B doesn't mean what you think it does," re-running Experiment B may have been a waste. How much consideration of these factors do you think a referee owes the authors?