A certain percentage of the population would stop working entirely since they no longer need to. (Particularly when politicians push the basic income amount up to placate constituencies)
Because workers bow out of the system certain industries lose productive capacity. Maybe we stop producing enough food, or enough fuel.
Prices rise (assuming a moderately free market), which causes the basic income amount to rise and it spirals out of control from there.
The state is forced to intervene. Either by forcing people to work certain jobs (ala https://en.wikipedia.org/wiki/Professional_Air_Traffic_Contr...) or by capping the basic income and reincentivizing work.
Hayek: The curious task of economics is to demonstrate to men how little they know about what they imagine they can design.