If a breast dissection if performed for the first time in the submuscular plane in the proper fashion, it would be almost impossible for implants to slip out from under the muscle to a plane over the muscle as all of the attachments from the muscle to the overlying breast tissue should be intact. There are techniques called a Dual plane 2 and Dual plane 3 where the lower portion of the muscle is detached from the overlying breast tissue in order to allow for more expansion of the lower breast. Even with these techniques, the muscle is still mostly attached to the overlying tissue and  should not be able to tear free. In some cases, if the muscle is extensively dissected it can retract in an upward direction where it does not cover much of the implant anymore. I have seen plenty of cases where patients had implants supposedly placed below the muscle, but by the time I saw them the majority of the implant was not being covered by the muscle because of muscle retraction or due to inferior migration of the implant due to skin stretch. Also, some patients I have seen for revisions have been both over and under the muscle, and in these cases it is possible for implants to shift back into the subglandular (over the muscle) position if that space was not effectively closed out when the implants were switched to an under the muscle position. In cases where muscle coverage is suboptimal, reinforcement material such as Strattice can be used to help bridge the gap and better support the implants.