Haestrom in Mass Effect 2. The planet's star Dholen is in the process of becoming a red giant which causes Haestrom to experience magnetic eruptions and increased solar output. Exposure to the sun's energy causes your shields to deplete rapidly, so you must try to remain in the shadows as much as possible, running from cover to cover.
There are also several planets in the original Mass Effect that have extremely hot, cold, high pressure or toxic atmospheres that will begin to overwhelm your armor after exiting the Mako. Metgos and Mavigon in particular have Level 2 hazards, which will damage your health within seconds of being exposed.