I once made a 10g sump but didn't realize that the silicone that I bought was white and not colorless. I was so eager to put the thing together that I went ahead and used the white silicone. Needless to say, it is without doubt the ugliest and most pathetic looking sump you will ever see! I'm half tempted to try to cut out the silicone, remove the glass and start over, but it doesn't seem like it's worth the effort at this point.
Not so much a mistake as overkill...
When I bought a used 135g tank for FW some time ago, it came with a stand that was falling apart. I opted to build my own. Being fearful of building something flimsy, I went to the opposite extreme. There were no 2x4's on the thing. It became a hulking monster of 4x4's and 1" plywood everywhere! Needless to say, the stand weighed much more than the glass tank empty and required a crew of people to move!