I have read about an equipment recall on aircraft due to errors caused by cosmic radiation at altitude causing problems with a specific sensor.

But I agree with you that this would much more likely cause unintended stops or stalls, or engine misfires, or computer resets, rather than just unintended acceleration on hundreds of vehicles.

This may be due to a failure mode, but it will likely be due to something like corrosion of a sensor or bus (as suggested by one professor) rather than single bit errors.

I assume from your description that the company isn't likely to go public any time soon, and that it is probably not likely to be acquired. (You'll want the agreement to cover those circumstances, but I don't think that's your major focus.) So ignore everyone above talking about going public, options, etc.

I'll bet that the company has a half-dozen to a dozen employees, and basically the owner / CEO is offering you a share of the company that they think you've earned over the last 6 years. With a half-dozen employees, figure his profits go down by 20% if you leave. (Until / unless he can successfully replace you.) And if you're good, and perhaps competing with them, it might hurt even more.

So basically, this sounds to me like you're becoming a minor partner in the firm. That's not a bad deal in theory. You get to be the owner of a consulting firm without having to deal with starting it from scratch, finding accountants, setting up accounts receivable, payroll, etc.

So follow up with a lawyer and a tax accountant to verify the deal is legit. And ask for company financials to get an idea of what 10% of the company will be worth on an ongoing basis. (i.e. how much would dividend you'd have been paid the last 2 or 3 years.)

But if you like the work, go for it!