I’m going to try some experiments with cardboard just to see how it works as a prototyping tool. But I can’t find any info on the forums or FAQ about what happens if I mess up the settings and end up hitting the crumb tray (or what’s below) with the laser. Can the laser mess up the crumb tray?
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The crumb tray is metal and can’t be cut by the laser, so you’re good!
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Don’t worry about it, you WILL hit the crumb tray 
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We cannot cut metal with the glowforge…
So on the down side, no cut-metal projects.
But on the up side, your glowforge cannot cut itself in half if you mess up the settings!
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When you cut through material, the laser often hits the ribs of the honeycomb and reflects a flash off onto the back of the material. That is flashback. So the idea is to just go through the material and the masking and not get much flashback. On acrylic it is especially noticeable that it flashes and creates lines back up into the acrylic on the edge so edge lit signs have artifacts of flashback. You can’t hurt the crumb tray. Dan recently pointed out that cutting without the crumb tray has its issues in that the reflection off the bottom metal plate can bounce out into places not meant to be hit with a laser.
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For acrylic especially it is often helpful to place a piece of sacrificial material in between the acrylic and the crumb tray. I use thin chipboard (which is a thinner version of what cereal boxes are made of but those work too).
Funny, I get more flashback from the GF than I do on the Redsail - both have steel beds. Not sure if the design pattern of the bed is what makes the difference as they are differently shaped.
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I use newspaper/coupons. I’ve found it works perfectly and has the added bonus of they keep sending me free replacements all the time. 
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Thanks for the answer @jbv, that’s right.
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