I was IFR at 4000 yesterday, the controller said, "fly hdg 280, maintain 3000." I read back, "hdg 280, down to 3," or something along those lines.
ATC: "say altitude."
"3200 descending to 3"
ATC: "how did that happen, sir? Climb and maintain 4000"
"you assigned me 3k and I read it back..going back to 4000."
ATC: "Well, I didn't get it...climb and maintain 4000."
I know the 7110.65 tells controllers to say "climb and maintain" or "descend and maintain" when issuing a higher or lower altitude not associated with crossing a specific point, however, _MANY_ controllers abbreviate it and leave off the 'climb' or 'descent' portion from time to time. Rather than question the instruction, I simply read it back and did it. If I questioned it every time the shortcut was used, it would make for some pretty laborious interactions.
As it turns out, there was a departure in the area climbing up to 3000. The controller quickly stopped his climb at 2000 to ensure separation until I got up to 4. The controller asked me to say altitude once I was at 4k, followed by a terse, "thank you," clearly thinking that I was still a moron.
Later on, he apologized for the confusion and acknowledged that he may have inadvertently assigned the instruction, and that he would go and check the tapes to find out.
This is a pretty subtle topic, however, the problems that it can cause are not so subtle. Under just the right circumstances, this could've caused a deal (if it didn't already), or worse, a mid-air.
So, to the question...what should I have done differently, if anything? The controller issued an instruction and did not correct my read back. Yes, the instruction wasn't per the .65, but as a pilot, I'm not expected to know the exact phrasing that ATC will necessarily use (coupled with the fact that the phrase he used is used on a daily basis by many controllers.)
The only thing that leaps to mind is a specifically worded readback such as "leaving 4000 for 3000," with a clear annunciation of the numbers to get his attention.