gimbal lock
Gimbal lock is what happens when a system that describes orientation with three separate spins suddenly loses one of its directions of turning. The classic picture is a set of three nested rings, called gimbals, mounted one inside another so that together they can point an object any way at all. Most of the time the three rings turn around three different directions and everything is fine. But if you tilt the middle ring far enough, two of the rings end up spinning around the very same line. At that moment you still have three knobs, yet they only let you move in two independent ways. A whole direction of motion has quietly vanished.
This is the price of describing orientation with Euler angles (roll, pitch, yaw). When the pitch reaches straight up or straight down, roll and yaw start turning the object around the same axis, so the math can no longer tell them apart and one degree of freedom is lost. In practice this shows up as a control system that jams or spins wildly when an aircraft, camera, or robot wrist passes through that pose. The fix is not better gimbals but better math: representations like the quaternion or the rotation matrix never line up their axes this way, so robots compute with those internally and only convert back to roll-pitch-yaw for humans to read.
The Apollo spacecraft's mechanical guidance platform could hit gimbal lock, so its crews famously had to avoid certain attitudes that would have frozen a ring.
A real hardware case where two rotation axes aligning erased a degree of freedom at the worst possible time.
Gimbal lock is a flaw of the three-angle description, not of the object itself; the object can still physically point anywhere, but the numbers stop being able to steer it smoothly.