unplanned
adj.happening or done without being organized or intended in advance. You use this to describe things that occur by chance rather than by a schedule.
adj.not conceived or organized in advance; lacking a prior scheme or intention. Often used to describe events, expenses, or consequences that arise spontaneously.
“The unplanned meeting lasted for two hours.”
“An unplanned power outage forced the factory to stop production for the entire afternoon.”
“While some critics viewed the city's growth as chaotic, others argued that its unplanned nature contributed to a unique and vibrant urban character.”
Typically used attributively before a noun, but can also follow linking verbs like 'be' or 'remain'.
From un- + planned.
Etymology adapted from Wiktionary, available under CC BY-SA 4.0.