One of the worst and most common errors in design is using many and incompatible fonts (type styles).

I think it is just too much of a temptation to designers.

I've seen a small card with 5 different fonts - and that was just on one side!

It's a distraction at best. At worst it prevents the piece from integrating. Viewers will wander off or feel like there's something wrong without, since they aren't typically trained in typography or design, having any idea why it bothers them.

It's so easy to avoid this error:

Resist. Temptation.