On XP 32 SP3
(PSD on PS CS5 : Maximize PSD File Compatibility is on "Always" - Generate Composite was OK)
After modification of 16b large psd file in LR 3.3RC
I get exclamation mark in filmstrip thumbnail
and "Sidecar File Has Conflict" in the balloon.
"Retry Metadata Export" not effective.
Have I reached some kind of limit?
I don't know what caused this error, but the direct answer to your question is NO, you have not reached some sort of limit.
I don't think there is such a thing as a too large .PSD file, and "16b" (whatever you mean by that) doesn't sound large to me.
Are you sure you have write access to the folder containing this photo, and to the .psd file itself?
16b stands for 16 bits /color and yes, the file and folder are acessible
(and I have re-read/re-written that file again with PS CS5 +re-tried LR before posting).
As visible in inserted image, dimensions are 15799x3609 ; File size is 1 222 033 KB.