A "kitchen table programmer" is a programmer you build at home on your kitchen table, i.e. a device built by a hobbyist without proper equipment, and experience to design, built and test it to professional standards.

The problem is that a hobbyist with lacking knowledge will often start from a bad design since he/she doesn't have the knowledge/experience to figure out that the design is bad.

Next thing is that the hobbyist will - just like most other people - introduce errors. Sometimes because a component is replaced with something that was thought to be similar - but true knowledge about the circuit would have indicated that the replacement component was not similar for a critical parameter.

Third thing is that a hobbyist with lacking tools/experience does not know how to locate an error in the home-built equipment. Especially if the tools are also home-built and may also be badly working.

Fourth thing is that a hobbyist without proper tools may use mounting methods and screening methods that are not working well. So one day, the home built (kitchen table) device may seem to work. Another day, one of the connectors has a bad connection, or the neighbour is busy using his WLAN network resulting in open input signals picking up the noise like antennas.

A fifth problem is that a lot of hobbyists posts their "designs" on the net. They post it because they have spent a lot of time on the projects and want others to see how good they are. But a lot of their designs doesn't really work. They run out of time before catching all hardware problems and before having written the full software that would have shown logic errors in the design. They also present their device is positive terms since they describe their "baby". A more critical presentation of a home-designed device might instead have spent more time on describing what the device can _not_ do than trying to convince readers about all the things it can do.

20 years ago, most hobbyists picked up their ideas from electronic magazines, or they bought books containing 301 different circuits. But most of the time, the schematics where designed by smeone who where skilled "in the art". And most of the time, the different designs also had some grading on how hard/easy the project was. Today, most people pick up their designs from random web sites with zero knowledge about the sills of the designer - or of the skills of the person who "presented" the design.

Another issue is that when people found designs in electronics magazines, they could also often buy component kits, including a PCB. This greatly helps a hobbyist to manage to get something to work. Especially if the magazine article also contains a bit of build and test instructions. Things like: Before soldering the IC:s, connect power and verify the voltages at the following locations: xxx