Having backups is always a good idea. Incremental saves for the win!
You can also hit Ctrl Z when you see things go astray.
Often it's easier to undo, fix the error(s) and then try again.
This can be easier to fix before than trying to fix the issue after - if you know what I mean.
Actually ,I don't advise you using Symmetrizing , it usually breaks model or make some parts of them breaks in term of both model and topology , Mirror modifier is much more better and effective , try to modeling while using mirror modifier is much more wiser and better.
zenogias First, mesh symmetrize works great as long as you have your mesh positioned correctly along the local origin. You may be thinking of the Topology Mirroring. Sometimes referred to as X symmetry. Mirror modifier is definingly better in my opinion. Second, This post is about Armatures, which don't have modifiers. It's X-Axis mirror can be buggy too, but the Symmetrize option under armature/right click context menu work great as long as you have the bones named correctly and positioned properly along the local Y axis.