During realm battles (versus AI wizards) I've noticed on several occasions that after an AI wizard is killed if it had any targets wrapped in web at the time of its death, those webbed units become impossible to free manually, you simply cannot attack the web that they are trapped in..
The ability to be able to attack the web to free your creatures doesn't exist from the moment the AI wizard dies.
Steps to recreate this -
Let the AI wizard web your mount (using its spider)
Kill the AI wizard so that the spider disappears (naturally all of the AI wizards casts will disappear.. except the web which gets left behind)
Now attempt to free your webbed mount with melee attacks
No attack % will be given
...I'm not sure if the trapped creature carries on auto-attacking the web each turn either.. so technically it could remain trapped, un-freeable (this part is unconfirmed though).