adventitious (adj.)

• accessory
• accidental
• additional
• adscititious
• aleatory
• appurtenant
• auxiliary
• casual
• causeless
• chance
• chancy
• circumstantial
• collateral
• conditional
• contingent
• dicey
• extra
• fatal
• fluky
• fortuitous
• iffy
• incidental
• indeterminate
• inessential
• nonessential
• occasional
• provisional
• risky
• secondary
• subsidiary
• superfluous
• supplemental
• supplementary
• uncaused
• undetermined
• unessential
• unexpected
• unforeseeable
• unforeseen
• unlooked-for
• unpredictable