This excellent PSA comes via the head honcho at TacStrike, Rob Tackett. The image below is a steel plate that had to be taken out of service far earlier than it should have due to excessive warping. As bullet strikes add up on a plate, it begins to warp with the outer edges bending toward the direction of fire. The warping shown in the image below is extreme and can only be accomplished via lots and lots of bullet strikes. This target has become unsafe to shoot due to the unpredictable way ricochets may come off the plate.
The simple fix to this problem is to turn your plate around and shoot at the other side for a while. This will halt the warping in one direction and begin to push it back into shape. You should be turning your steel targets at regular intervals. Consider it part of their preventative maintenance just like keeping them painted to avoid rust and greasing various hinges and bolts to prevent seizing.
All steel targets will warp like this eventually. It is important to choose a target that was designed by someone who understands this and has manufactured their targets to allow the plate to be turned easily.