Tombstone effect is when smt passive chip, usually resistor or capacitor, raises from another end yielding non-contact junction on lifted side of a pad. This happens especially with rework station while you heat the pads unequally. While reflow stage happens on another end, torque at that end lift ups the passive. This is how it looks like, a classic tomb stoning.
However, tombstone effect can be barely visible yielding faulty circuit, something to be aware if you are soldering smt components with a rework station.
Tombstone effect can happen another axis too. Instead of raising another end, passive component can rotate aside from the pad. Here is an example of rotated ferrite bead.
Tomb stoning can problem even with reflow ovens. Here is some variables to watch out.