Belt tension — the twang test
Press the middle of a belt with your finger. A correctly tensioned belt twangs like a guitar string; a loose one feels floppy. Over time belts stretch, and a loose belt shows up as ringing/ghosting (on CoreXY and bedslinger frames) or as layer shifts when the gears lose grip. Tighten the X and Y tensioners evenly, and check the belt teeth are not glazed or stripped.
Stepper motors skipping steps
A skipping motor loses position under load, showing up as a layer shift at a random height, often with a strained sound. Fixes: reduce print speed and acceleration in the slicer, confirm the motor current is within the driver's range, and make sure the driver is not overheating (check the heatsink fan and airflow). Both axes shifting points to the motor; one axis points to that axis's belt or motor.
Z-axis wobble
Regular, evenly spaced layer lines that repeat at the same heights across a tall print usually mean Z wobble: a dirty or binding lead screw, a loose Z coupler, or a motor skipping steps. Clean the lead screw and lubricate it with light grease, check the coupler is tight, and confirm the Z motor is not skipping.
Bed leveling
A bed that isn't level fails on the high side — the nozzle sits too far from the plate there, so the first layer is gapped and peels up. Print a bed-leveling test pattern; if one corner is high, re-level at all four corners. Check the bed isn't warped or loose in its mounts, and use auto-leveling if your printer has it. If the paper-drag test at Z=0 is consistent at all four corners, the bed is level.
Nozzle collision
A layer shift that starts right after the nozzle hits something — a warped print, a clip, a bed screw — is usually a collision, not a mechanical fault. Check first-layer height and bed leveling, make sure the print is not warping up into the nozzle path, and add a brim to keep small parts flat.
Extruder gear slipping
If the extruder gear clicks or skips while the nozzle under-extrudes, the gear is slipping on the filament rather than feeding it. Clean dust and smoothed-off plastic from the gear teeth, check the extruder spring tension, and make sure the filament path is not binding (a tight PTFE path or a kinked tube adds drag). If the blockage is inside the nozzle rather than the feed path, work the clog procedure instead.