Work as directed. If the sup's got a problem with the walls in a load you're cooperatively working on with another employee, you're both going to be responsible for the load unless your supervisor personally witnesses who's doing what. With that in mind along with the fact that you're still a probationary employee, the best thing to do with regard to job retention would be to say something along the lines of, "Yeah, I get it. I'll make sure not to do that, thank you."
Once you're in the union, you can pretty much just look at the guy and be like "Yeaaaaaaaah....
ooookay......" in response to this type of
.
If this is causing you unnecessary anxiety, try to talk to the new supervisor 15-20min before start time tomorrow. That could easily backfire, though, and despite your honesty the supervisor might be inclined to think you're full of excuses and just won't work for him.
As far as keeping your job, though, the absolute most effective way to ensure this would be to become a "Yes"-man by default, automatically volunteering for less desirable spots and more work, never letting it show that you're getting irritated regardless of how pissed off you are beneath the surface, and always taking the supe's word for whatever mistake of yours they point out, apologizing for the error and making a conscious effort not to let the problem repeat, whether it was truly
your mistake or not.
That's just for 30 working days, though. On your 31st you're in, and you can start dragging your feet like the rest of us.