The first time I saw someone had kicked out a power cord - while cutting their corner too close rounding a corner in a data center, the other redundant power supply kicked in and there wasn't a glitch in power delivery or downtime. The powering down of the currently primary power supply was logged however - so it did happen. That's how we catch those little events - even if you plug the cord back in quickly.
Why don't A90's / D90's have redundant power supplies, or log such events in the log server??!!
Most consumer devices don't come with an "idiot strap" to hold in the power cord - or any of the other socketed cords going in to the back of devices.
This is done to reduce costs and allow the "Darwin Effect" to play through.
Ethernet cables have locking lugs designed in to the socket, speaker wires that screw into posts will hold well enough to pull the equipment out of their position, and even RCA's grip hard enough for children to drag equipment by the wires, room to room.
There are positives and negatives around locking cords / cables, and overall it's better to have the cable pull out than the equipment be pulled down off their perch.
Usually the equipment survives the "unplugging" event just fine, as silence usually follows. The "dangerous" event is when plugging in a new AMP without reducing the volume controls to Zero before plugging the power cord back in...you did turn the volume's down before plugging back in?
The power doesn't change on minute by minute, second by second basis. The power is a constant, until the device is physically moved to another geographical region. There's no reason to burden the device with circuitry to detect power changes, set it and forget it.
In the past there were different sub-models used as solution - as there were different printed manuals - different power cables - etc - but that made inventory fracturing a problem - too many models. You'd get an order from a region without product on the shelf and they'd re-box them to fit, a costly and time wasting process. There would be overstock of one model and not enough of another model, as each region focused sales efforts.
Now multi-language manuals - or no manuals, all online - kits of plugs for power with power switches on the device - result in much lower costs for inventory on hand and much quicker response to fill sales orders.
The world is full of things we don't know and need to learn via our own research - or rushed mistakes - it's our choice which path we choose.
The D90 is likely just fine, reset all the volume controls down to Zero and plug in the power cord firmly this time, dress the cable with cable ties if you feel so inclined - you expect another potential incident to occur - that's why I "child proofed" my home racks for years.
Personal responsibility is in play here. Take responsibility for not plugging in the power cable adequately, or goofing some personal movement that jerked out the cord.
There's no manufacturer blame due here. It's all your responsibility.
If you did return the D90 the seller charge you for self-inflicted damage, and not provide free warranty repair or replacement.
Usually when someone pulls a boner like this the vendor laughs to themselves, and kindly replaces the user inflicted damage, if the customer is nice about it.
