Got a Dell PowerEdge T310 server. It originally came with 4 GB of RAM in the form of two 2 GB sticks. The sticks are ECC, registered, 1333MHz DDR3 RAM.
I bought two new sticks, each 8 GB, with the intention of upgrading total system memory to 20 GB. The new sticks are ECC, registered, 1066MHz DDR3 RAM.
The Dell technical guide for this system says you can mix modules of different speeds and it will run at the fastest common speed.
However, when all four modules are installed, the system will not boot. No errors, no beep codes, just no boot. If either set of sticks is installed, it boots fine. But put them together and nothing.