Weird problems caused by SFP DAC plugged into one switch but not the other.

We're doing a bunch of fairly major changes.
In preparation, I have been pre-running cables where I can.

I cause some problems (I cant use the word ou ta-ge?), because I plugged the SFP's into one switch, and left the other side hanging.
I'm new to SFP's, and am not sure if this is indicative of a bad SFP, or of a "NO YOU CAN'T DO THAT" moment.
The switch they were plugged into is a Dell 5548.
Basically the TG SFP interface was flapping, going up and down constantly, and I think it overworked the poor old switch, causing it to stop l3 routing on some of the interfaces.

So, is it a bad thing to plug in one end of a SFP DAC and not the other? or do I have a bad SFP?