Solved
Interference in thin films is a standard 1st-year physics topic. It would be nice to be able to label applets I create about it with this standard name for the topic. However, both thin-film and thinfilm are forbidden as tags. What is going on here? What am I supposed to call this topic? It belongs to the larger class of interference phenomena, but it's a distinct sub-category. This is the name every one of the tens of intro textbooks I have gives this topic.
The same problem 
I guess it has something to do with the antispam engine.
Thank you, I'm looping your question to the Support Team.
Film was on our spam list for tagging, so it wasn't possible to use this tag, therefore you couldn't use this tag. We have changed that, so now you should be able to use the tag thin-film.
Thanks! I've tagged my activity with thin-films now.
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