difektiĝi

A clean build, and a useful one to take apart. Difekti is transitive: somebody or something damages a thing. Add -iĝ- and the thing becomes the subject and nobody is blamed: difektiĝi is to get damaged, to break down, to go wrong of its own accord. La akvo difektis la libron is water damaging a book; la libro difektiĝis is the book ending up damaged, with the water left out of it.

That swap is the single most useful pattern in Esperanto verb morphology, because it replaces a whole English mess. English handles it with word pairs (break / break down), with a passive, with a reflexive, or with nothing at all. Esperanto has one suffix. The companion -ig- runs the other way, making an intransitive verb transitive, and both are set out in igi and iĝi.

Do not confuse the result with rompi, which is to break in the sense of snapping something into pieces. A difektita machine may look perfectly whole and simply not work. Difekto is the fault itself, and sendifekta is the word on a second-hand advertisement meaning undamaged.