HTC One sound problem
There are a lot of users that reported an annoying noise coming from the sound system of HTC One, especially when the user is using headphones. The static noise is more often heard when the music is paused or when the tracks are chaning, in the pause between them. Also, popping noises have been reported when a notification sound plays. A possible fix for this problem is to turn of Beats Audio option from the notification panel and to turn of the NFC option. HTC has not release any statement regarding the sound issues encountered in HTC One, so it is still unknown if this happens because of the software or the hardware. As an aditional fix, you can try to use a different pair of headphones and see if the noise is still there.
Capacity buttons problem
In some cases, HTC One’s capacity buttons will not work properly, and when you are trying to use the phone the buttons will not light up. In order to see if there is a hardware problem, you could try to cover with your hand the proximity sensor located in the upper left part of HTC One. If you see no light on the buttons, you may experience a sensor malfunction. Also, you could try to download Android Sensor Box and diagnoze your smartphone.
Unresponsive buttons
This is probably very annoying, since most users are using those two main buttons found under the screen a lot. There was no oficial statement regarding this issue, but I have few possible solutions it: you may try to touch the display few times before using the butoons(this may improve the buttons), you can try to calibrate them from the menu(Settings - Language & keyboard - HTC Sense input - Advanced - Calibration tool) and try to slightly touch them while calibrating and ultimately you may try to remove the screen protector(if you are using one).
Dead or stuck pixels
This is a common issue found in LCD screens, and since the HTC One is using a 4.7? Super LCD3 capacitive touchscreen, from time to time on of those pixels may get stuck on a specific color(red, blue or green). If the pixel is dead, you can do nothing about it. You may try to send the phone to the service and use the warrany, but I’m not sure they will replace the display just because there is a dead pixel. So, in this case, you’ll end up with a display that has a tiny black pixel. If the pixel is just stuck on one of those colors(red, green, blue), you can try to use a stuck pixel remover from Play Store. If you are lucky, it will come back to life using that app.