I am using Magento 220.127.116.11 in one of my site. I need to update New Security Patch SUPEE-7405. Will there be any issues occurs, if I update the new security patch?
How can we resolve those issues created after updating the security patch?
Bill installs a new security patch.
Bill notices that security patch causes unwanted side-effects.
Bill makes rollback using version control.
Bill is smart.
Be like Bill.
Security patch is like any other update you add to your store. First try everything out on your staging / test environment. If it doesn't work locate specific problem; there shouldn't be any, but, well, s*!t happens now and then and you can't avoid it completely. Consider it a known unknown. Post specific problem here or search for error message from Internet.
Magento is complex, the servers it will be installed on are so varied that each install can be totally unique.
Add to that the fact that it's open source so is hackable by everyone from raw amateurs to seasoned web developers with absolutely no server expertise at all and it's a total crapshoot as to what can go wrong.
So, places like this end up with multiple, searchable threads so you can hopefully find an answer. Other places like StackExchange will have someone post a question and then everyone gets to watch the answers flow in as here => http://magento.stackexchange.com/questions/98220/security-patch-supee-7405-possible-problems
If you are installing any security patch solely on your live ecommerce website without checking first on a development environment, your business plan includes loss of income stream by software failure. I've started to find it amusing that many online businesses have failure as part of their business plan.
Upgrade your business plan to include at least a computer that runs VirtualBox with a Linux VM you can use for development testing. It will pay for itself the first time your website doesn't stop functioning due to any sort of messup whether it's a misguided Magento mod attempt, third party module install, first time template install or Magento security patch.