Default risk, Inflation, Interest rate risk.

Default risk refers to the possibility that a borrower may be unable to repay a loan or other financial obligation; this can impact all 3 sources of financing since it would reduce the amount of money available for investments as creditors attempt to reclaim their funds. This could increase WACC due to higher interest rates being charged in order compensate for any losses incurred—which may limit opportunities or require additional debt funding if necessary.

Inflation is another issue which affects all types of financing by decreasing purchasing power over time. This means that what you are able to buy now may cost more down the road, resulting in increases in costs for businesses which could raise WACC as well (if funds remain stagnant).

Finally, interest rate risk is associated with changes occurring in borrowing/lending markets—which can have an effect on both equity and debt investments. A decrease in rates usually leads to lower WACC while an increase has the opposite effect; so volatility here can cause uncertainty when trying to calculate overall capital costs and make future projections etc. In summary then default risk, inflation and interest rate risk are all potential issues which should be taken into consideration when determining weighted average cost of capital—as they can have varying degrees of influence depending on current market conditions.