One of the most (unfortunately) memorable was when Mike Mussina left the Orioles to sign with the Yankees in the late 90s. Certainly left a real bad taste in the mouths of baseball fans in Baltimore, and marked the beginning of the 13 year slide.
Another would be the carousel of NFL teams that feigned interest in moving to Baltimore, without a realisitc shot of that happening. This hit its peak when Paul Tagliabue awarded an expansion team to Carolina and then took the coward's way off the podium by announcing that the other team would be determined at a later date. This cemented that Baltimore would not be one of those cities, despite having the best deal on the table.
Honorable mention goes to the Colts skipping town in the middle of the night, though I was only 5 years old at the time. Therefore it had no emotional impact on me, though I cannot imagine how heartbroken this city must have been. I'm glad I was not old enough to understand, but I certainly do now.