One of the first things that people who experience zero gravity for the first time do is start swimming. During the "making of" video for the OK GO film clip for Upside Down Inside Out they recently released, where they filmed inside a reduced-gravity aircraft (a "vomit comet"), they all observed that the first thing they did when exposed to zero G was flail around like idiots trying to swim around, as if they were dropped in water. Unfortunately, as the air around them provided very little resistance, they generally just flailed around in place.
Definite puking. The feeling of zero gravity really messes up your sense of balance and your body responds to that by think it has been poisoned, and ejecting stomach contents. Again to use that video as an example, there were nearly 60 instances of vomiting during the filming.
Heart attacks would be very unlikely, as reducing gravity would take stress off someone's heart in the short term. 978
Muscle strain would be unlikely as well because it's easier to propel your limbs when they don't weigh anything. Breaking bones would be possible if they lose control of themselves and carom into things. Going weightless results in the equilibration of bloodflow around the body, and as such would not make anyone faint.
Long term effects of zero gravity tend to be things like muscle atrophy and related heart problems, as well as loss of bone density, but these take months to years to manifest. Your eyes also change shape after a long period of time in zero gravity, but will return to normal when exposed to regular gravity within days to a week.
Note that these things really only apply to people who aren't used to zero gravity. If you have experienced operators, they will be able to switch between different gravity areas quickly. Dropping the gravity from 1.0 to 0.2G, for example, would make people go from running normally to bouncing around like they did on videos from the moon landings, except they would not have more than 100 pounds of suit attached to them and could get even higher with their jumps. 461