Hi Dave. Regarding your statement above, can you please point me to some documentation? I'm not questioning your claim, but I've always done it the other way and recommend that others also do the same. If I've been wrong all these years I'd love to be able to read something verifying that. Thanks
AFAIK, An AC to DC Transformer will draw AC even when there is no load, and after unplugged the capacitors will hold a charge for quite a length of time, so realistically I'd argue that in a humid environment you can't really prevent sparks. Mind you, I don't have any documentation for that myself, just my observations of AC to DC Transformers which get warm and emit coil whine when there is no load suggesting they are still drawing power from the wall, and how the device they are attached to will sometimes light it's LED for a fraction of a second when attempting to turn it on when the wall wart/power brick is unplugged, but won't when it isn't (which tells me it's not, say, internal capacitors in the device). I also have no idea whether this would go towards plug first or device first.
I myself have never really paid any mind to what order I plugged bricks/wall warts in.
As far as leaving it plugged in all the time, this can lead to another problem in which the laptop battery is always charged and never goes through a drain/charge cycle and so the battery life for the laptop can be lessened as the battery dies prematurely.
Anecdotally, my Toshiba Satellite L300 from February, 2008 has a battery that is now 8 years old. I seldom power the system from battery, particularly now that it has been replaced as my primary laptop. However it can still power the system for a little over 2 hours, which to me seems to be better than I would expect given it's age (it originally powered it for about 3). My understanding is that Lithium-Ion batteries don't get the same benefit from being discharged as other battery types and aside from excessive charging voltage the main consideration would be storing the battery, in which case discharging it is preferred. Even so- High temperatures have a greater effect than the charge.
At any rate the way it's been explained here makes me think there is something else going on like perhaps poor grounding, as I can't see a device sparking such that it damages or leaves burn marks.