a bit hard to troubleshoot. Might be the HTC charger issue maybe it degraded, or electricity spike, or might be the extensions' socket degraded. Was the extension socket in good condition?

Coz i got 1 extension, 1 of the socket damn scary, whenever i plug in anything (charger, kettle) into the socket, I hear like electric buzzing. 

Any thunders last night? Penang area got a lot of thunder storm in the past 2 days, which could burn your electronics easily.

Err...no lar...

I think the fuse in the charger would likely cut off the power to the phone when it was overloaded, possibly due to aging or power surge. Sometimes the devices got shortcircuit as well if the surge is too strong or fast. The fuse minimize that from happening but its not a 100% thing. You got lucky.

Ah yes, your explanation sounds more logic. Sorry for my silly explanation earlier.

Let's see if I remember this correctly, read somewhere that if the charger/adapter output is greater than the device , then it should be fine . May not be optimum and may cause damage internally (ie. Battery, circuit) if prolong usage.

DANGER : output is lower than device recommended specs. This will cause the charger to work 'harder' to meet the difference . Hence, may cause an explosion if prolong usage.

Best is to use the device's original charger .

I'm sure your extension plug comes with  fuse. So if there is a spike on the incoming, the fuse will get burned first.
In your case, it might be not the incoming current spike, could be the HTC adapter internal circuit kong (possibly overcurrent) and heat up+blow the adapter. This happen if your adapter Amp rating lower than the charging Amp of the phone. In new adapter, there is a built in protection circuit to protect the adapter from overcurrent and getting hot+burn. Old adapter, I doubt so. You are lucky it is not your phone. By the way, can you run the Battery Monitor widget and see what is the charging flow when you connect your phone to a 2A charger? On the earlier threads, we found out the Mi3 pulls max of 1.7A for charging.