Another common error is only using hreflang on domains that have a dot com. This is incorrect.
I have heard this excuse from dev teams that cannot develop a solution to map pages across different top-level domains. They assume since they use ccTLD’s like .co.uk, the search engine should understand what country they are targeting. For more on ccTLD’s and their benefits check out Eli Schwartz’s excellent SEJ article. Unfortunately, search engines interpretation of ccTLD’s are not perfect, and I recommend doing a test to make sure it is understood and that no other local sites are ranking in the market. If it is working great, but if other market pages are ranking, then you should make the case to implement on all domains.
Clarification: It is strongly recommended that you use the hreflang element for any page that has an alternate no matter what domain it is on. This is actually one of the best reasons to manage your hreflang elements with xml site maps. Enterprise hreflang xml tools can map the URL’s no matter what domain they are on. You can also host the xml site maps for all of the different sites in the same location. This makes maintaining them much easier.