My pet peeve -- or maybe it's a weakness -- is when the customer asks a ""simple question"" about an integration. They say that you don't need to become involved as it will be easy.
Then over the next few days, weeks, months the inquiries keep coming and at some point the tables are turned and somehow the story the customer believes is you sold, recommended and promised that this integrated solution (that the customer purchased on their own) would work.... then it becomes ""why won't you"", ""why doesn't MAS90"", ""don't other people do this"" ...