Used wrong bubbles
A long long time ago, in a galaxy far far away, I failed a "whiteboard" interview, given a scenario & had do a dataflow diagram. I asked the correct clarification questions and got the right result (they used a small system they developed and I matched the analysis document 95% ), but still failed - "oh we use Gane & Sarson symbols (rounded rectangles for process) here, and you used the Yourdan symbols (circles) , so thank you very much for your time".
Gob was well an truely smacked