As suggested by a user on reddit. Perhaps a warning or an error bar?
Maybe add a similar bar to the one used on Piggybanks, it stays orange until you go over your budget which it then goes red. This would be a clear visual when going over budget.
I'm not sure how easy it would be but including it on the Budget page, but also when adding a transaction. So you select the budget on the transactions and it then loads the bar showing current spent + this transaction.
I've knocked together a quick example on how the bar could look on a Transaction Screen
Bar Example:

Trxn:

And if already over budget:

And if the transaction will take you over budget, then the blue element of the original bar could be red.
That is a very good idea!
Most helpful comment
Maybe add a similar bar to the one used on Piggybanks, it stays orange until you go over your budget which it then goes red. This would be a clear visual when going over budget.
I'm not sure how easy it would be but including it on the Budget page, but also when adding a transaction. So you select the budget on the transactions and it then loads the bar showing current spent + this transaction.
I've knocked together a quick example on how the bar could look on a Transaction Screen
Bar Example:

Trxn:

And if already over budget:

And if the transaction will take you over budget, then the blue element of the original bar could be red.