Disclaimer
Safety
Modifying your vehicle can affect its safety and road worthiness. Consider the case when you pull out of an intersection and your engine dies because a resistor rattles loose. Consider that there is a truck coming and that he doesn't see you. I rest my case. If you modify the electronic control systems on your vehicle or retrofit them to replace outdated technology such as carburetors and distributors, you should do so to the highest possible standards. Settling for less than the absolute best quality of work is unnecessarily risking your own life, and those of others. In fact, even if the hardware designer, whoever they may be, chooses a particular component and it turns out to be significantly too light-duty for the application and fails leaving you in front of that truck, do NOT come knocking on my door asking why.
Legality of general modifications
Depending on where you are in the world, modifications of various sorts can be totally forbidden and illegal, right through to legal without consultation with the authorities. In New Zealand for example, on all vehicles sold new or imported before the first of January 2008 that are not turbocharged and are originally fuel injected, you may, without a modification certificate, do as you please to the existing engine provided that basic safety is not compromised and you do not increase power by more than 20%. That includes swapping EMSs. On turbocharged vehicles though, no changes may be made to the electronic or mechanical control systems without an inspection and modification certificate being obtained. Most parts of the world are far more strict than this. It is YOUR responsibility to ensure that what you are doing is legal where YOU are. For example, if you slap a turbo on the side of your Daihatsu Midget II and get pulled over, arrested and locked away, do NOT come knocking on my door asking why.
Legality of modifications regarding emissions systems
Depending on where you are in the world, modifications that affect vehicle emissions can be totally forbidden and illegal, right through to legal without consultation with the authorities. In New Zealand for example, only vehicles sold new or imported after the first of January 2008 are not allowed to be modified, and must pass specific exhaust emissions tests. All other vehicles only need to pass a visible exhaust smoke test. I believe the USA state of California is the most strict in this regard. Once again, it is YOUR responsibility to ensure that what you are doing is legal where YOU are. For example, if you slap a turbo on the side of your Toyota Tercel, tune it to 10:1 AFR belching out black smoke and get pulled over, arrested and locked away, do NOT come knocking on my door asking why. From another member of DIYEFI.org : "In California, people have been known to fail a smog test for using blue hoses where black ones were OEM. Having a car impounded is not fun, can be quite expensive, and it will significantly "de-fun" the experience of making your own EMS to see your car crushed."
Aircraft
Because of the old adage "what goes up must come down" (and accelerating at a rate of 9.8m/s^2) it is unwise to use experimental DIY fuel injection systems of any kind on aircraft at all. I expressly recommend that you do NOT use any device from this site on something that flies. If you ignore this advice it is at YOUR own risk. For example, if you fit a FreeEMS to your Cessna and it fails because of a poor solder joint at 10000 feet and you plummet to the ground and end up one-legged, do NOT come knocking on my door asking why. In fact, if it fails because a developer forgot to check for divide by zero, you don't want to have the lives of your passengers on your conscience either, do you? 'nuff said.
The point
The point is that while everyone does the best they can, it is all experimental and there is a reason it costs $300 and not $30,000. Use your head before building anything you find on this site. If not, you may end up regretting it. To quote another member of DIYEFI.org : "I'm not responsible for anything, you are responsible for everything."
GNU GPL Warranty and Liability terms
For software obtained from this site.
15. Disclaimer of Warranty.
THERE IS NO WARRANTY FOR THE PROGRAM, TO THE EXTENT PERMITTED BY
APPLICABLE LAW. EXCEPT WHEN OTHERWISE STATED IN WRITING THE COPYRIGHT
HOLDERS AND/OR OTHER PARTIES PROVIDE THE PROGRAM "AS IS" WITHOUT WARRANTY
OF ANY KIND, EITHER EXPRESSED OR IMPLIED, INCLUDING, BUT NOT LIMITED TO,
THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR
PURPOSE. THE ENTIRE RISK AS TO THE QUALITY AND PERFORMANCE OF THE PROGRAM
IS WITH YOU. SHOULD THE PROGRAM PROVE DEFECTIVE, YOU ASSUME THE COST OF
ALL NECESSARY SERVICING, REPAIR OR CORRECTION.
16. Limitation of Liability.
IN NO EVENT UNLESS REQUIRED BY APPLICABLE LAW OR AGREED TO IN WRITING
WILL ANY COPYRIGHT HOLDER, OR ANY OTHER PARTY WHO MODIFIES AND/OR CONVEYS
THE PROGRAM AS PERMITTED ABOVE, BE LIABLE TO YOU FOR DAMAGES, INCLUDING ANY
GENERAL, SPECIAL, INCIDENTAL OR CONSEQUENTIAL DAMAGES ARISING OUT OF THE
USE OR INABILITY TO USE THE PROGRAM (INCLUDING BUT NOT LIMITED TO LOSS OF
DATA OR DATA BEING RENDERED INACCURATE OR LOSSES SUSTAINED BY YOU OR THIRD
PARTIES OR A FAILURE OF THE PROGRAM TO OPERATE WITH ANY OTHER PROGRAMS),
EVEN IF SUCH HOLDER OR OTHER PARTY HAS BEEN ADVISED OF THE POSSIBILITY OF
SUCH DAMAGES.
17. Interpretation of Sections 15 and 16.
If the disclaimer of warranty and limitation of liability provided
above cannot be given local legal effect according to their terms,
reviewing courts shall apply local law that most closely approximates
an absolute waiver of all civil liability in connection with the
Program, unless a warranty or assumption of liability accompanies a
copy of the Program in return for a fee.
TAPR OHL Warranty and Liability terms
For hardware built from documentation from this site.
7. WARRANTY AND LIABILITY LIMITATIONS
7.1 THE DOCUMENTATION IS PROVIDED ON AN "AS-IS" BASIS WITHOUT
WARRANTY OF ANY KIND, TO THE EXTENT PERMITTED BY APPLICABLE LAW. ALL
WARRANTIES, EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO ANY
WARRANTIES OF MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE, AND
TITLE, ARE HEREBY EXPRESSLY DISCLAIMED.
7.2 IN NO EVENT UNLESS REQUIRED BY APPLICABLE LAW WILL ANY LICENSOR
BE LIABLE TO YOU OR ANY THIRD PARTY FOR ANY DIRECT, INDIRECT,
INCIDENTAL, CONSEQUENTIAL, PUNITIVE, OR EXEMPLARY DAMAGES ARISING OUT OF
THE USE OF, OR INABILITY TO USE, THE DOCUMENTATION OR PRODUCTS,
INCLUDING BUT NOT LIMITED TO CLAIMS OF INTELLECTUAL PROPERTY
INFRINGEMENT OR LOSS OF DATA, EVEN IF THAT PARTY HAS BEEN ADVISED OF THE
POSSIBILITY OF SUCH DAMAGES.
7.3 You agree that the foregoing limitations are reasonable due to
the non-financial nature of the transaction represented by this
Agreement, and acknowledge that were it not for these limitations, the
Licensor(s) would not be willing to make the Documentation available to
you.
7.4 You agree to defend, indemnify, and hold each Licensor harmless
from any claim brought by a third party alleging any defect in the
design, manufacture, or operation of any Product which you make, have
made, or distribute pursuant to this Agreement.