egg with no shell

The other evening one of our hens, Wyandotte bantam, laid an egg straight onto the concrete outside the pen and it was just like someone had cracked an egg onto the floor minus the shell. she has been laying normal eggs for quite a while now and this is the first time its happened. It happened the same evening, Wed, our cockerel went to his new home. So would that have caused it or could she have got stressed as I had accidently shut the gate of the pen and they couldn't get in 
Or is it just completely normal for this to happen. There was no sign whatsoever of a shell.
Anne