1. Ignoring mobile
So you optimized search for your website already? How does it look on mobile? Ensure your pagination uses enough white space for touch, ensure thumbnail images are clear, and scoping tools are easy to use.
Newegg's a mobile-specific site (note the m.domain), yet the search box, thumbnail images and product links are lacklustre on iPhone.
Doesn't Reebok wanna take advantage of showrooming??
2. Zero results found
Allowing zero results found is a captain obvious mistake, but serious -- and very common even on today's top ecommerce sites. Great search tools handle mispellings or attempt to provide something relevant to the query (partial match).
We've covered in detail 4 constructive ways to handle no results found on the blog before, if this is an issue on your site, I encourage you to check it out.
3. One result found
Far more dangerous is sending the user to your "top hit" for a search term. Why?
1. It gives the impression that there is only one result, while at least with zero results found the customer may try a modified search. (Even Google would ask if you were feeling lucky before serving such a feature!)
2. A user will never see your full selection matching the keyword. You're banking all your search conversion on one result. Would you do this to your category pages?
I recently searched for "bronze" on a shoe site and it sent me to a product called "Bronze Age" which did not come in the bronze color. It happens - even on large brand sites. Don't let it happen on yours.
4. Missing sort / filter tools
Category pages typically include great refinement tools, some quite creative.
But search results pages benefit from these tools just as much! Make sure they're replicated on your site to improve usability.
5. Search box field too small
This is an old web usability tip from Jakob Nielsen, but all the more important today with mobile part of the mix. Longer search box fields allow the user to see their input and reduces errors. How long should the box be? Check your search logs to see what length of search query is typical.
6. Google Custom Search
If you're not careful, your site search could be advertising the competition...
7. No scoping tools
Scoping tools are not necessary on every ecommerce site, but for brands, manufacturers, telecoms and software publishers for which site content goes beyond the online store, the ability to search and scope between product results, site help, documentation, etc. is critical -- and often missing.
The Microsoft Office site promises search results from the "entire site."
But it forces support results. Huh?
The user has to figure it out that product results are hidden behind that obscure call to action that is the Store link in global navigation.