All in all, quite an interesting worked example of what is wrong with our politicians and media. If the pun can be forgiven, "ready, fire, aim".
If a system could be guaranteed to be 100% reliable, there would be no need to test it. For such a critical system, nobody could possibly guarantee 100% reliability, so testing is necessary. Arguably the most valuable tests are the ones where something goes wrong (as long as nobody is injured obviously), as a point of failure can be identified and eliminated.
There was no risk to life or limb here, other than to any passing fish, and there would be no danger posed by a live firing going astray.
For me the biggest disappointment was the Prime Minister's inability to state the bleeding obvious: Love Trident or hate it, the debate doesn't hinge on a single test failure. The failure in question was and is absolutely irrelevant to the debate regarding nuclear deterrence.
Finally, and the point has been made already, it is dumb to trumpet successful tests on you tube etc. This sort of activity should be conducted discretely, which would have created a more accommodating backdrop for occasional failures.