This is a connection-related bug where if you begin to lag after having leapt off a platform, low or high, or even if you were hopping along as TheLiquor mentioned above, the server attempts to predict where you are despite the lag and consequently predicts that you’re still mid-air. On the server side, you could be perhaps 100 metres underground and hitting something which ultimately kills you. Improving your connection would help (perhaps see if there’s an improvement by enabling the ‘Small Network Packets’ option) but nonetheless it is an issue on our side and we’re looking in to it!
Sorry, I know I said I wouldn’t bore you with the details… 