the old answer is "Finding half a worm"... but as we experiance more, there are several things worse than that:

Warning: don't read if you are squeemish!

When I was in my early teens in a caravaning holiday, we baught a mince round for tea out of a buchers. Fresh new potatoes, mince round and garden peas... yum.
So we all sit down to eat, mum as usual last to sit down and start eating (since she dished up ours first and we don't stand on ceremony) We had eaten about half of the serving of the pie when mum lifted the crust of hers and found some little white grubs: maggots. Cooked, so they weren't squirming, but still not the best thing to find when you've already eaten half of your share.
I would say that's worse than the 'half a worm'.

Any other tales of grusomness?