HP-28 series

[1] A year later, the more common HP-28S was released with 32 KB of RAM and a directory system for filing variables, functions, and programs.

This model (and the HP-19 series that used the same clamshell design and internals) had one design problem that affected many units: the springs inside the unit that made contact with its three N-sized batteries made a considerably strong force, and the notches on the case in which the lid edges slid were too thin and shallow.

Even if the case itself was sturdy enough, those notches were under extreme pressure, most especially while the user was opening or closing the lid (such as during the replacement of the batteries).

As the lid edges were made of a harder metal, the plastic notches in the case were prone to cracking or breaking, even in carefully kept devices.

[2] Surviving examples of the earlier versions of this calculator frequently have rubber bands around or tape over the cover to hold it in place (including the image attached to this article).