Title pretty much says it all.

I got her killed when I unknowingly walked into a cave with a Master Vampire

Boethiah

She probably died of boredom & loneliness, since I never took her out of Breezehome.

Who is Lydia?

Poor old Lydia...

First time was she ran in front of a Shout, and got blown off a cliff along with a bear...

Second time she ran in front of a power strike, and copped my magic axe of killing in the back of the head, instead of the Draugr it was intended for...

A couple of other times it was because of the cavern full of Draugr Deathlords...

The latest, and this was an absolute cracker that cost me about half an hour of gametime looking for her, was when I used the Stormcall Shout fighting a dragon, and she kept getting hit by lightning and then staggered around and fell into a river, and got washed all the way from up by Riverwood to down near Whiterun. Great Shout, but sadly no good around friendlies...

The fire storm spell. I did it twice before I realized she's just an idiot and she will run into a battle even if I tell her to wait somewhere, unless it's through another door. The other time I killed her she ran into a power strike. Apparently she doesn't like the whole "axe in the face" thing. Who knew?

She got so sick and tiered of my arrow in the knee jokes that she took an arrow to the knee.

Boethia's quest or Ebony Blade, depending on whichever I feel is more pertinent on that character.

Mine died of obesity from eating so much bread because I left her in Breezehome.

Got jumped by Spiders.

I didn't make this video but it made me laugh..http://www.youtube.com/watch?v=xZjehEPbun0 and immediately did the same thing to Lydia in my game. hahaha!

she died ignominiously when i fought kamatu and his group of alk'iri warriors. I got tired of having companions die on me so I have givin up using them.

She died at Northwatch Keep on my Orc warrior playthrough. I've recently started a second character as a Dunmer mage, she died in Ladyrinthian.

A huge undead Kinight character with his own powerful Shout just kept fighting and wouldn`t die. I had to run to a safer area to try and archer it to death.

After being repeatedly knocked down, Lydia died.

I found her body among the numerous undead a bit later...

and then she became a guard for whiterun am I right?

She got in the middle of a disagrement between my self and a Deathlord. He shouted, I shouted, Lydia died. unfortunately 2 minutes later so did I.