Please tell your customers if you are out of a certain ingredient. My LHBS didn't have c60 so they took it upon themselves to "make" c60 by giving me half c20 and half c40 because "its basically the same thing".
I now only shop there if I'm desperate for time and need something that will take a few days to get from Northern Brewer.
Half c40, half c20 would be c30!
I will never forget the time that a friend and I were touring a fairly well known midwest microbrewery and the tour guide was sharing a story about how they knew exactly how much grain the grist-case held:
"Well, one day, we ran out of Crystal 80, so we decided to just use twice as much Crystal 40, unfortunately it was more than the grist case could support and the chains holding it buckled..."
We slowly looked at each other, didn't say s***, drank our free sample beers and high-tailed it out of there.