I'm a US citizen living in Spain with my EU husband and am waiting for Spanish residency and it's taking longer then my allowed 90 day stay. I was told to stay in the country while I wait. However, is it safe to travel to the US for a short trip while I wait? If I bring the documents that I have that prove I'm in the process in case I get stopped in the US or Spain. Need advice, and can't find a straight answer.
Who told you not to leave Spain while the residency application is pending?
Others that have done this before, saying it's best to wait until you get it....
"Best to" is not the same as "must," of course. Legally, when you arrive at the Schengen border, if you can prove that your EU-citizen spouse is in Spain, or anywhere else in the Schengen area or the EU, and you are traveling to join him there, you must be admitted (subject to very limited exceptions related to public safety, public health, and public policy). The same is true if you are traveling with him. This right is codified in Directive 2004/38/EC.
The documents showing that you've applied for a residence card should not be necessary, though they probably would be useful as evidence. More useful still would be copies of the documents you submitted with the application.
The reason it's "best to wait" is probably that arriving at the border and having to go through the process of proving this could be time consuming and stressful, and there may even be a possibility of failure. It may also be that your application would be considered abandoned, forcing you to start again from scratch. I think it works this way in France (nominally, at least), but I don't know about Spain.
The only problem I could think of is that you stayed more than 90 days in the Schengen Area and the border guards think you have overstayed but as phoog pointed out correctly, the documents you submitted should prevent problems at the border.
Specifically, that should be a translated and apostilled or EU issued multilingual marriage certificate and a copy of your husbands residence card or its application.
You may on the other require a visa to get back in, especially if airlines check or ask about your previous Schengen stays. However, the Spanish consulate at your place of residence in the US should issue such a visa. That visa should be free of charge and once they accept your application is in my experience (with Spanish authorities) a matter of hours.
That is of course, unless you got something like temporary residence document that attests your status in Spain.
New Schengen visas (not extensions) can also be issued while already inside the Schengen Area, though that might not necessarily apply in your case, as while in Spain only consulates of other member states would do so, and only if you intend to travel there with your husband and not giving you the visa would restrict your husbands freedom of movement.
If in doubt, carry a copy of the Schengen Handbook with you.