I've actually solved this, but I think its still a useful question which I don't think is easy to answer with existing questions.
As soon as I'd built my ANET-A8 (Prusa i3 DIY kit), I found I was having problems with the extruder crashing into the bed. Although I thought I'd adjusted the bed leveling OK, the calibration seemed to keep getting messed up.
I tracked this down to two factors. First, I was winding the extruder head up some distance before loading the filament and starting a print. Second, at roughly half-way up the axis, the right-hand thread seemed to be getting stuck (more often when moving up than down).
What wasn't clear (and not mentioned in the building instructions) was what might cause this problem.
I assume you did everything according to the instructions but here is a checklist of what could be possibly wrong:
- Friction - check if you can rotate/move parts without lot of resistance
- Screws - check if screws on couplings are tight and they don't slide over a shaft or thread
- Stepsticks - check if they are cooled properly and similar (as there are two of them)
- Carriage nuts on threads - check if they do not slide out of their nests while [the x-axis] carriage goes up
- Filament - check if filament unrolls without resistance which can eventually cause [the x-axis] carriage to hang.
IMO #2 and #3 are the most possible cause