Colbert Bump: Inverted when Jay Leno permanently took the lead over Letterman in the ratings after his 1995 interview with prostitute-solicitor Hugh Grant. This came in the wake of Letterman's disastrous stint as host of the Academy Awards that year, where his style of humor clashed horribly with the tone of that event.
Two more of Letterman's personal favorite artists, Warren Zevon and Phish, also gave the show two of its most famous moments with their last appearances on the show — a show in 2002 in which Zevon was the only guest (it was his final public performance before dying of cancer, which he had been diagnosed with a few months before appearing on the show) and Phish performing a multi-song set on the Ed Sullivan Theater's marquee in 2004 (only one song was taped for the show, but in true Phish fashion they just kept going after that).
Seasonal Rot: Dave has been on broadcast TV for almost 30 years. In the mid-2000's, the quality of the Late Show has declined slowly. However, every so often, Dave will have his standout moments, such as the infamous Joaquin Phoenix interview or the mileage he got from the Jay Leno-Conan O'Brien controversy.