Class-M planets, various
In the Alpha/Beta Quadrants, one otherwise unnamed world among several Class-M planets in the Mar Oscura Nebula winked out after showing up on sensors, then blinked back, a result of the dark-matter spatial deformations. A sensor scan and three probes revealed no life nor anything unusual.
In the Delta Quadrant, one barren world, despite its rating, is nicknamed "Planet Hell." It turned out to be the home of a reptillian spacefaring race.
Another was small planet some three hours away from the point where Janeway left Voyager for the Tak Tak world, populated by Garan miners who contracted a mutating macrovirus; the Tak Tak "purified" it by killing all those infected.