There's no logic. We see a brand that will work in one and not in another of the same model. Other brands will work, but its hit or miss.  Been that way for years (I've been on this forum since 2002). 

The only reference most of the techies on here use is what Crucial lists for a specific model.

Unless you were knowledgeable about Dell memory quirks it would be hit or miss.  

FWIW memory compatibility is not limited to Dell.  I had a desktop with an ASUS ROG motherboard.  I had some Corsair memory and some GSkill memory - both same specs.  The Corsair worked OK by itself and the GSkill worked OK by itself but with both installed it wouldn't boot.

Dell's have a history of compatibility with non-Dell branded memory.  Some brands will work in one and not in the other.  Mixing memory is another problem.  Only have all the same brand/type.

We suggest Crucial memory as it is guaranteed to work in a Dell.

You didn't note which version of 780 you have.  Here is the Crucial listing for a 780 desktop

Dell OptiPlex 780 Desktop | Memory RAM & SSD Upgrades | Crucial.com

I have a mini tower.

Yes, I have heard that crucial is compatible, but my question was not related to that. I was wondering if there are any resources that I could have used before I bought my RAM to know that it would work. What specification of the RAM that I bought is incompatible with the dell optiplex? Crucial is not the only brand that works. My previous sticks were Micron mt8jtf12864az-1g1f1, yet I can't find any detail or spec that would have helped me differentiate the compatible from the non-compatible. 

Thanks for the quick reply. To be clear, I am not inserting mis-matched sticks of RAM at the same time. The samsung M378B5273CH0-CH9 does not work by itself. 

So, are compatibility issues on a brand to brand basis? So if I buy Non-ECC unbuffered 4GB Micron sticks, should I expect them to work in the optiplex? 

Is there a list or forum where people share what sticks/brands work for the optiplex 780, or other old computers in general? PartPicker does not go back far enough in history. 

780 is not memory picky.  As long as it is ddr3 non ecc desktop only unbuffered unregistered 1333 or 1600 it should work.  Have had a few 780 just bought used ddr3 on eBay to refurbish them (mostly mt). Never had menory incompatibility issue.

3-1-2 Master DMA register failure

try replace cmos battery then clear cmos settings via motherboard RTCRST jumper.

The M378B5273CH0 - CH9 memory is listed as 1.5V memory. The Crucial memory I linked is 1.35V memory.  Could be why it doesn't work.

@redxps630 reports no compatibility issues, which surprises me.

I don't know of any 780 mod site. You will have to search the web.

It uses standard desktop ddr3. Ddr3L would also work.

When buying RAM, I've simply used something like "Optiplex 7010 RAM" for a search term in Amazon.

A Tech Guide or User's Manual can't possibly list all the model #'s of RAM a PC can take.  It will tell you speed and which DDR version though.

Optiplex 780 Tech Guide.pdf 

Optiplex 780 Service Manual.pdf 

@redxps630 Apologies, I wrote the beep code incorrectly. It is 1-3-2, not 3-1-2. Besides this, I have tried changing the CMOS battery, and reset the RTC. These did not work.