Budgets should be reviewed on a regular basis to ensure that money is spent within limits. In model standing orders and financial regulations there are limits on what committees can spend without seeking approval from full council - in my council it is £5k.
In essence, nothing will happen really but there may be the requirement for monies to be transferred from another budget. It should be raised at Full Council and the reasoning discussed and a solution decided - whether the committee is disbanded, gets its knuckles rapped or the overspend approved.