Difficult to judge from a forum post, and I know from your usual contributions on here that you come across well, but it looks like there may have been some lack of prior communication with this first band. You have worked with them before so may have thought you don't need to go through their expectations. When I went in with my last band we worked with the studio owner/producer to work out what we hoped to achieve on a modest budget. We took his advice and it worked brilliantly. Our guitarist even had time to do a couple of fabulous, unplanned overdubs.
As for the OK/cool thing, I can see how that could be misinterpreted. I know it sounds daft, but how about coming up with a slightly more formal, neutral phrase to indicate the end of the recording?
But yes, from my music shop experience, there are some clowns out there!