What are the perverse effects of sales promotion?

Basically promotions are the only way to stay in front of customers and prospects. The biggest benefits of promotion is branding so automatically they know your brand and product. 

Ads can have a both positive and negative effects people, human psychology is very different with everyone so care must be taken in account when devising such promotions and strategies as to not create a negative impact. It is also likely that people might feel that your product is not doing well or that your product is only worth if purchased at a discount, a few other thoughts can come into play here but what is important is the feedback, every critique is important for you as that will help you strengthen your weak areas.

That people might just be interested in buying your goods or services only when it is in promotion price. 

thanks for the invite!