If I have no job, I get $25,000? What if I have a job making $15,000 - do I get $10,000? If I have a job making $24,999 I get $1?
If we assume everyone who currently makes less than $25,000 (after taxes) takes the money and quits while everyone who makes more than $25,000 keeps the job (a faulty assumption, to be sure, but it works for this) what does it do to the economy? Obviously nearly all food service jobs, retail, etc... are vacated. Why work for less than minimum guaranteed pay? Those that kept their jobs now have difficulty spending it for groceries and restaurants. Obviously that's not a sustainable position, the market will correct by forcing wages to go up on those low income jobs until there's enough people to fill them. Obviously the prices on those items also goes up to pay the now required higher wages to fill the jobs. I suspect we'd see a shift to lower labor alternatives. It'd make more sense to buy a few weeks food at a time at a grocery store than buying fast food for very many meals.
We'd also see the same thing that happens now on all income contingent welfare benefits/disability. If I make $10,000 on a job it reduces my $25,000 to $15,000. But only if the $10,000 job is known. A lot of low income jobs will probably shift to "under the table". Cash doesn't count as income, right?
Some of the downsides could be avoided by making the guaranteed $25,000 constant regardless of income. Whether you have a job making $10,000 or get a salary from daddy's corporation of $2,000,000 pay them both the $25,000. The $10,000 minimum wage worker then has the choice of $25,000 or $35,000 so there's still incentive for them to work. To be sure, some would decide the extra $10,000 isn't worth it. But the effects would be less severe.