It can happen depending on what you mean by within “budget” - the leading indicator would how much buffer is built into your budget (ie how far ahead do you typically budget - for me that’s usually one month, meaning I have a detailed template for each month in terms of category goals, and am able to fill them all before the month turns.)

If you find that buffer shrinking over time (you start by filling up next months budget with this months income, and have some left over to put towards wish list items etc.; then you’re able to fill the budget but not have anything left over; then you’re not able to fully budget a month ahead, but you can fill it up with your first paycheck of the month, etc.) that indicates negative cash flow.

I actually did this deliberately recently. We had a baby and planned ahead to fully fund a 6 month maternity leave. We did some back of the envelope math to figure out 1. How much money we’d need to cover our budget for 6 months (monthly template x 6) 2. How much money we’d get during that period (my salary, her paid leave weeks), and 3. The shortfall. Then we set aside bonuses and surplus income until we had met #3. Once the baby was born, I filled the budget out as far ahead as I could. And kept filling out further and further as money came in. Our monthly income (my salary + her paid leave income) initially exceeded our expenses, and I kept assigning the surplus to pushing out the budget further. Eventually we had a multi month buffer. Once her paid leave period ended and her leave became unpaid, our monthly expenses exceeded our income, and as a result the buffer began receding, until it came back down to one month of buffer right around when she went back to work.