TL;DR: If the game won't accept your save name, just add a space to the start of it!

So, I just acquired Dungeon Hack, played through a bit, went to save my game and... it wouldn't save. Just kept blanking out my save entry. I couldn't figure out what was going on and accidentally crashed the game trying various things out. Eventually, with some effort, I figured out what's going on...

See, I go by the handle "Gemini" typically. I tried various iterations of my name and discovered "gem" was what was keeping the save from succeeding. I thought about this a bit more and it occurred to me: The game is deliberately blocking certain words from being used at the start of save names! :O

After some more testing I found out all checks are limited to the first three letters of the save and may even be two letters or just the very first character, thus you can completely bypass this issue by entering a space as the very first character for your save file. As for which three letter words are blocked from the start of the save file name, I've discovered these so far:
"gem"
"meg"
"tim"
"som"
"red"
"hum"
"ke"
"<" (Yes, even adding a space before this character lets you make an actual " <empty>" save!)

It'd be interesting to see if anyone can figure out any more of these. ;)