Mario was indeed, complicit in the crime

When Corin was in the bathroom with mario and she was crying she said others in the house knew it was Sam Pepper hiding her stuff and never told her I  was disappointed Mario made out he knew nothing about it.

Tricky situation that one.
If every housemate spoilt a prank or intervened to reveal one in the Big Brother house it would be bedlam.
I am sure Mario would have intervened if he felt things were going too far.
Mario and many housemates in all BBs have been pranksters.
Anyone who spoils that fun runs a big risk of becoming rapidly unpopular.
I think in this case if Mario had spoken out it would definitely not have put an end to it.
The campaign to target Corin had gone too far for that.
I think a watchful eye was probably best.
But this is not a new thing.
Always been pranksters, always will be. But if they were always sabotaged we would be disappointed.
As we know Mario likes pranks, but this is one he didn't want to be hands on

not much different to what he did to ben.

The trick is being a fun housemate and supporting pranks, but then knowing exactly when to call a halt. Difficult balance.

If you see somone is being bullied ,you call a halt,if it is indeed a harmless prank you say nothing and support the prank.

Apparently his socks aren't clean, either.

Not for me it wouldn't be...

Mario was fine with it all until last nights task .
He did not like or agree with the categories he put into on the board,now he is vocal.

In real life it's often much more clear cut.
But in the BB house?   It's different. It is virtually programmed for pranks, right from the production team down to each housemate.
Big Brother will have pranks.
I would guess that almost every housemate has been aware of pranks planned on another housemate.
This is not just Mario; it is all housemates.

How do you know erinp?

I can see how he thought it was a prank at first, (which then went too far.) I can also see how he might have felt intimidated to stand up to them when he knew it had gone too far. BUT I think that the very least he should have done was own up to Corin, (who is supposed to be his friend,) that he did know about it, but thought it was a prank, then apologise. Corin certainly won't be buzzing when she finds out!

He didnt tell Corin what was going on ,he didnt ask Sam to stop.

I acknowledge your points Super. 
I have to leave now for a while.  Thanks for discussion. I like you all 

I think sometimes we forget that the HM's don't see everything we see. Maybe Mario wasn't aware of how far it went.

Having said that..... any HM who didn't stick up for Corin today is cowardly, but Mario is certainly not alone there

Nobody's hands are clean., but he's the least offensive in the whole debacle.