When a Firework is spawned either with summon command, from a dispenser or by placing it on ground and if the player dies immediately after. If you stay on the death screen for a while, the client freezes. Sometimes it recovers and you don't have to restart MC. The problem is worse the longer you stay on the death screen. If you click Respawn straight away, then everything is usually ok.

To reproduce this bug make the following redstone machine:
1, place pressure plate on ground
2, place command block with command "kill @p"
3, On the other side of pressure plate put a command block with firework summon command such as:

/summon FireworksRocketEntity ~ ~ ~ {LifeTime:20,Motion:[0.00,0.00,0.10],FireworksItem:{id:401,Count:1,tag:{Fireworks:{Explosions:[{Type:1,Flicker:0,Trail:1,Colors:[2293504,10895615],FadeColors:[16777215]},{Type:4,Flicker:0,Trail:1,Colors:[16716820],FadeColors:[16777215]},{Type:1,Flicker:0,Trail:1,Colors:[2959103],FadeColors:[16777215]},{Type:4,Flicker:0,Trail:1,Colors:[16713942],FadeColors:[16777215]}]}}}}

Now do the following:
1.Stand on pressure plate
2.When you die, wait on the Respawn screen for more than 10 seconds.
3.Now click Respawn. The client will probably hang for a while or freeze completely.

This bug is worse when playing on a server but seems to still happen in single player but not as bad(client more likely to recover).