That's why you have a friend help. They can point out the eighteen ways it can go wrong. I mean, yeah, you could go it alone, and the internet is helpful, but it can turn out exactly like you say. I've helped friends with home projects where it turns out a trip to the hardware store is in order. More times than not I'll mention that "it looks like we'll need a whattzit and a hoozsnoggle while we're there" because the need for those items won't become evident until you come back home.

Underestimate? No, I know how awry things can go. In our first house I noticed a leak in the basement under the toilet above. Wax ring, easy fix and cheap. Except it had been leaking a while, meaning a good piece of the subfloor had rotted. Up comes the carpet (who the hell thinks carpet in a house with men is a good idea in the bathroom?), might as well put in new linoleum on top of the new sub flooring. Since the floor's coming up, there's no better time to put in that sink the wife wanted. Somewhere in the process we discovered the wet drywalling under the shower stall. To the Depot for new drywall and shower stall! Weeks later I finished the job.

All because a sixty-nine cent wax ring failed.