I've touched a soundcard once. It started squeeling like a stuck pig into speakers. I just shut down the system. The other time i touched something on graphic card and it just locked up.
No biggie or just pure luck. But in general i prefer to power system down, flip PSU switch to OFF and press main POWER button on PC case to discharge capacitors (fans usually spin again for the last time). Then it's ready to work without any problems.
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