Usually a player won't worry about this, since so long as the budget is in surplus they'll return to positive next turn.
BUT if you have a launch scheduled that turn and it has any problems, the game then asks if you want to hire tiger teams to solve it. You say "hire no tiger teams" since you can't afford any... and it tells you that you can't afford to hire 0 teams. The game is then stuck there indefinitely, you can't do anything to get past it.
Evidently the system checks to see if the tiger team cost is greater than current funds. Since 0 is greater than even -$1, it gets stuck.
I would suggest one of two solutions,
either - don't let the player go to negative funds through sending people for study, etc
or - have the "no tiger team" choice not bother checking against current funds.
I imagine the first is the simpler solution to implement. In the meantime, I just go back to my savegame and cancel the offending mission
