Not the first time it happened and it will not be the last time. I caught several planes myself with this mistake (fortunately it never happened to me so far, knocking on wood).
The system is not Murphy-proof. After installation of the new / refilled oxygen cylinder, the valve has to be opened fully and then closed by one quarter-turn, after which it gets lockwired in this position using copper wire.
If the engineer/mechanic gets distracted while doing the installation, he can easily foreget to open the valve completey. Usually by the time he walked up into the flight deck the lines have the full pressure, so he will not notice anything unless he does a oxygen mask test for at least a minute.
Often engineers/mechanics do the job alone and have nobody there to crosscheck.
This is why it is so important for you pilots to do a proper oxygen mask test, watching the gauge for pressure drop.