I was a doing a set of 130 lb bench presses but loaded the bar wrong and had 5 lbs more weight on one side than the other (my weaker side fwiw) and it was a lot harder than usual. So I was wondering if unbalanced lifts would be good for breaking size/strength plateaus?

I wouldn't do it with lifts where you are standing up, personally. You tend to compensate for imbalances naturally and in some parts imperceptibly, and it's much easier to lock up muscles than to relax them once they've tightened. You could learn bad form and perhaps screw up your posture somewhere down the road. I guess it wouldn't matter with seated curls and such.

Make sure your diet/sleep are in check; this can do a lot with plateaus. Also try switching up your routine. If you've been doing the same thing over and over again and it's not getting you anywhere consider changing something up.

no it's a horrible idea, mostly because it makes no sense. You could injure yourself too. Is the issue that the weight jumps are too big? Look into micro plates, which allow you to add as little as 1lb to the bar.

If you're plateauing, you either aren't putting enough stress on your body to stimulate progress, or you are too fatigued to show your true strength. Give us more detail about your training history in particular your recent progress. Also let us know how you have felt.

Basically two polar opposite paths to take:
1. Take a light day to let yourself recover. This will dissipate fatigue, which should allow you to show more strength.

2. If that doesn't work, do more volume (you can spread it out over multiple workouts, for instance a week)

Mostly I suspect that you might not have a good structured program with a plan for progression.