Investor should be very cautious being long shares of T-Mobile US Inc (NASDAQ:TMUS). Rarely, have I see such an intact, perfect bear flag formation on the stock chart. Chart setups this perfect rarely do not play out. A bear flag is formed when a stock has a sharp rollover, then spends a long period of time inching sideways and slightly higher. Ultimately, making up approximately 50% of the sharp down-move. This is exactly what has happened to T-Mobile, and investors should take note. This tells me there is a 75% chance or better of another big drop coming, with a downside target of $55.75. From the current price of $63.47, that would be a drop of 12%.