I Have seen this Problem since 7 months ago and have not seen any change.
I have my shield up and when a Jelly or any other monster attacks me i take damage.
My sheild has got me killed like this!
I hardly not take notice cause i got this on film in a Vanaduke run i did.
http://www.youtube.com/watch?v=BAW6HEmG-J8
2:32 - 2:40 is the Glich that happened.
I had my shield up just before Vanaduke Charged and i got hit.
This is Happening alot!
I can even have a Sheild up sometimes 5 Seconds and take a Weird hit.
That is enough evidence which is Visual to what Bug i have seen since December 2011 to July 2012.
Please Do look into it.
PS: LAG is also a Cause of it i know, But i did not lag the other day when this happened.
In the Film from Starting it up i lagged and a little while latter it wasn't so laggy.
I saw it coming and acted.
Latency is your issue.
Everything your character does is server-side, your button presses tell your character on the Three Rings server how to behave. Which is what mainly gets on people's nerves, especially in lockdown ("Lag" is what people call it). Vanaduke acts in his normal fashion while you appear to have about half a second delay between button presses. This may come from a variety of things, mainly a poor ISP.
I recommend upgrading to a better internet provider for a lack of a better suggestion.