A mixed shelf offering indicates that United Airlines may issue new securities in the future, which could include common stock, preferred stock, warrants, or other financial instruments. This can lead to dilution of existing shares, which typically puts downward pressure on the stock price. However, the impact may be mitigated if the market perceives the potential use of proceeds as beneficial for the company's growth or debt management. The lack of details on the size of the offering adds uncertainty, which can also negatively affect investor sentiment in the short term.