Once upon a time they added Snarby to the game. We had to use bells to kill this boss. Then they made a Shadow Lair version where you have to fight two of them? Easy right? Yeah I just did a Shadow Lair Snarby and was reminded of the rage that is known as the bell not stunning the Snarby when its in range. Seems this bug still isn't fixed since this boss was added. Also I believe it only happens when you try to stun a Snarby that is attacking though a Snarby is basicly always attacking.. but I have noticed a handful of times where I stunned a Snarby and it either burrows underground stunned, or doesn't get stunned at all and keeps going with its 5 bite attack.
That is all.
Happens™