Regression Testing --Assumptions are worst enemies

How many of you are like me thinking regression testing is the easiest part of testing? Ha ! I dare not anymore! I was thinking on those lines until I encountered a major issue in the system that I tested  minutes before it was about to be moved to production. This happened years ago, when I was in the initial 'fresher phase' (<2 years)of my career. Things do not seem to be as good as they ought to be.  I was testing an application in one of my earlier companies. We did not have any formal team,and each of us were assigned a module to test. I was more than happy with the module that I had to test as it was full of bugs :) what more can a tester ask for ? everywhere you click you see an error, and nothing worked! I logged bugs in 100's and was pretty confident on my work. Then came the fixes, with a scintillating Ux. Logging 100 bugs is easy, but trust me, verifying 100's isn't that easy. I verified all bugs and completed few more rounds of testing, the bug count reduced gradually and everything seemed to be good and neat. With just a couple of hours before production,came the final build for regression. When there were literally no bugs in previous version, little did I realize that I would encounter any major issue.I assumed that what worked earlier, would work fine in that build as well.I could hardly think of any issues after working so hard the whole month, everything was looking good to me. Stuck in front of system and a piece of module for a long time, did take a toll on my thinking ability. I could not think of anything else, and literally signed off the testing for my module. The go-live process started and each of us were so confident about the modules that we tested. With just minutes to go, to get the help file, I logged into the system in QA environment. I wanted to check my module one last time. Ha! the submit doesn't work!!! what a crap, how can it be, immediately cleared all cache, changed browsers, did all things that I could do, the submit just doesn't happen! Group chat on, submit not working!!! Wow, the first thing that any user would do is try to submit something and it doesn't work! Go-live plan put on hold, investigation on,.. The last time I remember testing submit was one build back, ok, then bring back that build , test it on QA, it worked! The last build has this issue, so what was the reason, a simple suggestion for alignment changed the button position and the functionality stopped working! Would you believe it, yes, that was the issue, reverted back, and it worked again! All were happy, they all termed it as "Minor glitches" in production. But I just could not excuse myself for taking it so easy. My assumption of things would work the way it is supposed to, proved me wrong big time A lesson learnt .. Regression - Toughest part of any testing activity.