Android (rooted): Many Android phones have a popup that nags you whenever you turn your volume up to a certain level. If you're rooted and have Xposed installed, you can disable this with a simple tweak.
The message -- which usually reads something like "Listening at a high volume for a long time may damage your hearing" -- has good intentions, but can be annoying. We're not advocating destroying your ears with high volumes, but there are plenty of times where maxing out the volume is necessary. Maybe you've hooked it up to a portable speaker, or maybe you have some higher-impedance headphones that are harder to drive. In those cases, the high volume warning can get really old really quickly.
Luckily, it's easy to turn off. Install Xposed (if you haven't already, it's awesome), open the Xposed Installer, head to the Downloads section, and search for NoSafeVolumeWarning. Install it, activate it, and reboot your phone. The next time you try to crank up the volume, that annoying message won't bother you.
Note that if you're using GravityBox, this option is already baked into GravityBox's settings, so you won't need to install the NoSafeVolumeWarning module. Just enable it under the "Media Tweaks" menu.