beer yes not sure about lager
It certainly happens with beer, collect drip trays (if not scummy) and pour into a bucket and filter back into barrel. No barrel tampering required, the filter funnel fits through the spile hole. I didn't personally do it but did see it done.
Line cleaning is a simple process though you really shouldn't be getting line cleaner flavoured beer! I guess if you are busy you could possibly forget to pull through water before beer.....
(cmouse is pondering about life in other jobs, Fri 8 Jul 2011, 10:31, 2 replies)

It depends
If they're running around like coked up spaniels due to under-staffing (as the staff at the local 'spoons always seem to be) then management might (particularly if not real ale drinkers like the guy who tried to argue that the malt vinegar taste in my mate's pint was "normal for a real ale, they tend to be punchy") be inclined to skip, or not be as through as they should be with, that part.

That said caustic soda probably would do you sufficiently bad damage to be a worry from a litigation stand point...
(Rounded Rycroft Thought of something witty but posted this instead, Sat 9 Jul 2011, 16:48, closed)

It was definitely done with lager
Fosters, Carlsberg and Kronenberg, all thrown into the same barrel and served up as 'Fosters'. I'm not saying there weren't complaints made by customers; the policy was no refunds, exchange only. If they moaned they could have it swapped for a pint of strongbow (which wasn't tampered with).
(TheManWithThePlan cussed your mum on, Sat 9 Jul 2011, 19:31, closed)