many many ways.....
The Millenium falcon had a typo on the plaques. Instead of "Cannon", they printed it as "Canon".
The Shock Trooper and Spec Ops helmets, they claim to have "lost" most of the limited edition plaques....so majority of the helmets were recalled and from that point on no numbered plaques were issued with the helmets, only ones with the total edition size for example "Limited Edition of 750 Worldwide".
The ObiWan TPM Elite Editions suffered from major QC issues. Lots of people had excess glue residue all over their sabers, and some shoddy construction where parts were loose, broken, or damaged.
The Vader Helmets were shipped out with crappy QC as well, a lot of them had scuff marks in the dome and badly painted around the eye lenses and mouth area.
All of the ROTS sabers don't come with the standard type of plaque which has both the number and total edition size on it. After customers complained MR started issuing second plaques for free which has the total edition size on it. This is why you see the ROTS sabers being displayed with two plaques sometimes.
The Dooku sabers had problems with the rubber grips peeling off after awhile. The glue that MR used must not have been good for long term. A lot of the sabers have grips that are falling off or have fallen off completely.
Just some things off the top of my head