Android OS. I swear, this thing does the most irritating shit on a daily basis. A lot of times I'll pull google up to search for something and I'll type out the first two letters and my whole phone will freeze while it's cycling through possible words to auto-complete for me. I'm not talking about freezing for a few seconds; this thing will completely lock itself for upwards of a minute. So, being the incredibly smart guy I am, I turn auto-complete off. It will still lock itself, though. I have no idea why, as it shouldn't be auto-completing anything. I dread having to actually use Google on this thing.
It also randomly won't unlock. I swear, I'll be swiping that button for 30 seconds while pressing the power button on and off while it stares at me. I'm pretty sure that this phone thinks I'm an asshole.