Because Airbenders live at such high altitudes, I assume they have naturally strong lung capacity, allowing for some natural defence against overinflation. Waterbending healing is a subset of Bloodbending, manipulation of the body's water. Thus, it would be possible for a healer to mess up a wound. Katara's accidental healing could be evidence of an instinctual correct use of bending to avoid accidents, including overinflation or overheating.
Earthbending causing bruises? Maybe for amatuers, we've seen people bite rocks into pieces, smash them into bits, and shove their hands into them. If an earthbender were inattentive, perhaps they could injure themselves.
Overheating or frostburn seem like the most likely injuries.
Multishipper: Taang [favorite]; Kataang; Zutara; Sukka; Sokkla; Tyko; Toko. Never really liked Tokka, used to like Maiko before season 3.