I once read the following tidbit about the development of Mahou Shoujo Madoka Magica, but I cannot recall the other anime involved, and I cannot find a source mentioning this.
As I recall, one of the writers or producers of Madoka was watching a certain anime as it was being broadcast, and it influenced them when they either wrote the script for Madoka or just got a certain key idea between two episodes. This person later remarked that, had they finished watching the anime before writing anything, the plot of Madoka would have been much different (possibly less dark?).
Is this true, and, if so, what was that other anime?