Bad ideas in the workshop are not just about things that don't work ... or things that are a waste of time, they also include things that are potentially dangerous or can cause injury, and often, it can be a combination of bad things.

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I'm going to start off with something that I have rarely seen, but it does seem to happen with a few new woodworkers ... 

For some reason, I have found that some newer woodworkers seem to have this compelling need to remove bits of wood from around the table saw blade after the saw is turned off but the blade is still spinning. If you see this you NEVER want to raise your voice to them in case you startle them and they end up moving their hand into the blade ... and of course, we all know this is something that should never, ever be done .. by anyone ... but the fact is that sometimes it is.
Where this is leading me is why I do NOT APPROVE some guards over some bits and blades that could inhibit someone from pulling their hand away quickly from an area of danger. We never want to make a safety device and area for concern and some of these do that. 
As woodworkers, we all know we must ALWAYS wear good quality eye protection so putting a clear Plexiglas cover over something like a crosscut sled is really just a redundant level of safety. If it's your sled and you are confident only YOU are going to use it and there is so absolute reason you need to do this ... then go ahead, but otherwise, the guard could turn out to be problematic.
And for another thing, just the fact that you may need to add "stops" at the back of the crosscut sled from time to time is the second good reason that this kind of safety guard will get in the way of that. 

Table Saw Sled
Here's another example of what can happen when making jigs ... I know ... I made something like this many, many years ago and found that worked extremely poorly. Why? ... because after you make the jig, and when you first make it the jig performs as it should.
But, as you continue to drill holes, the twist bits slowly eat away at the side of the pilot hole making it bigger and bigger which means you start to lose the alignment of the holes and as this happens the pilot holes continue to enlarge. Where this is most problematic is if you are attempting to make adjustable shelving that requires all the holes on all 4 sides to be identical, otherwise, your shelf wobbles or won't even fit. 
The best solution for this is pegboard sheeting and Vix or centering bits which will not enlarge the holes of the pegboard.   

Self Centering Drill Bit

There are times I wish I had a 20 or 24-inch jointer and plane, for those times I need to flatten a long wide slab of wood.  Below is a mock-up of a setup I have seen that uses a couple of long pipe clamps as edge guides. I have used long pipe clamps over the years for clamping and I know they bend so when I saw this, I wanted to see just how much bending there would be, so I again mocked up this set-up to test it. 

Wood Router Sled

With a measuring stick precisely on the top of the router, with no downward pressure, the stick barely touched the top of my router. 

Next, I pushed down on my router, which is what you need to do when you are hand planing a slab because otherwise, the router bit rides up out of the wood. As you can see there is at least a 1/8th of an inch gap when I do this, even a 1/16th would be unacceptable. 
When this happens, the bit is cutting grooves into your slab instead of flattening, and what's worse at the ends of where the clamps are attached to the boards there is no movement, which means you are going to end up with a slab groves all through it and lower in the middle than the ends.
All in all, just a bad idea. 

Wood Router Sled

Here's another bad idea that continues to float around the woodworking community, and that is making your own filler by mixing sawdust from your sander bag (which is very fine) with wood glue. Now, this is not a bad idea if you are going to PAINT over top of the filler, but if you are expecting to stain or dye the wood, or even just apply a finish like an oil or poly finish, this is a bad idea.
The reason this doesn't work is the glue is absorbed into the wood sawdust and it makes the sawdust waterproof or nearly waterproof, which means dye and stains will not absorb into the filler. I made a video about this some time ago detailing this and if you missed it, you might want to check out that video HERE

Wood Filler

The good news with bad ideas is that it makes us come up with better ideas that do work, share them, then we all learn together
Copyright Colin Knecht