So, I was trying to print this model on my Prusa i3 at home. Went to bed about 2 hours into a 9 hour print and was thinking, “Dang! This is the cleanest print I’ve ever done!” Seriously, it was 70-80mm high and looking pretty… Woke up this morning to this stringy mess. I’ve had prints fail overnight before, but I’ve never had one unravel what had already successfully printed. Any ideas on what happened?
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…mystery solved 
Found this sitting under my printer desk. Got knocked pretty far off the build plate. Looks like the edges curled up, hooked on the nozzle, and the rest is exactly what you’d expect.
Another reason not to leave your printer unattended 
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I’ve had situations like this before, for a piece like that you would probably want to have a good z jump in place so it doesn’t snag and snap off the arms as they build.(or knock it off)
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Yeah, I’ve only recently started messing with retraction. Haven’t messed with z jumps, but I’ll have to try that! This has happened a couple times, almost always on taller prints
Maybe worth using an enclosure to keep it from curling.
What material are you printing in? What are you using to prep the bed.
Hair spray on glass works well enough for me that if it’s ABS or PLA it is stuck pretty well.
Next on my list to try out 
I’m using PLA till I can build an enclosure with filters. I’ve used hair spray with great success, and I’ve also used blue tape on glass.
On further inspection, I found a slightly melted section of cable wrap on the cable bundle going to the extruder (didn’t damage wires). Seems that the bundle dropped below the hot end and got pinched between the hot end and the piece. Quite lucky a bad print is all that went wrong! Printed a cable chain pronto, and problem solved. I’ll try a z jump next… Once I’ve finished an enclosure
If its curling you could actually use more air flow on the nozzle as well as the z hop settings others suggested to avoid snagging any curling that does appear. Depending on your slicer you might be able to set minimum layer time giving the previous layer more time to solidify before adding the next layer which would also help reduce curling.
PLA does not do well if you keep it warm. It needs to solidify faster than abs so in this case an enclosed build chamber may not be better.
I printed a T-Rex with supports. Hindsight it didn’t need them. I spent several hours flossing a dinosaur.
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The lower jaw prints as you see it in the picture, the upper part of the skull prints with the nose in the air. Both pieces printed in glow in the dark PLA, without support and I scaled it up as large as I could.(It was a great Christmas present for a dinosaur obsessed 7 year old)
This print was optimized by someone at makerbot to show off what makerbot could print, it prints great once you’ve got a setup that bridges well and does overhangs well.
Here’s a tiny one, this was around the 3rd print to ever come off my printer
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