An objection.
Physical danger from the environment, especially with risk of drowning
An obstacle that hinders achievement of a goal.
The state of being difficult, or hard to do.
An awkward situation or quarrel.
That which cannot be easily understood or believed.
An antipattern.
A potential, unsuspected, hidden problem, hazard, or danger that is easily encountered but not immediately obvious.
A type of trap consisting of a concealed pit in the ground, which the victim is supposed to fall into and not be able to get out from.
Subsidence below ground in a mine, which can cause the ground level above to drop.