Unexpected output from 74AC244 buffer
You have a floating input pin, which is picking up noise and causing erratic outputs. The added impedance of the scope probe on the input is enough to stabilize the input. You need to add a pullup/pulldown resistor if you are just using an SPST switch.

Also, note that if the actual device driving the buffer has an output that can drive both high and low, you will not need a pullup/pulldown. If, on the other hand, it's an open-collector output or similar, you will need a pullup.