problem with lines placed on different quotas

[arkvalery]

work with autocad 2011, I was quietly working on my plants (domandandomi why the command "gather" did not work anymore), however wanting to accinger me to realize a 3d I passed to the assonometric view and I realized that many lines were on different quotas... How can I return the lines to the same quota and especially how can I avoid this happening again? had never occurred before, maybe I have some wrong setting? Thank you very much for your help:

 

[Tristan]

if you have installed express tools you can use flatten command
to avoid similar drawbacks you can set to 1 the osnapz variable