I was expecting to see two of my pet hates - returning a null array to represent no items, and returning a single object without an array to represent one item.
I also once worked with an API where you had to send the data in POST format - abc=123&def=456. After much pressure from their customers, they finally relented and added an XML version of their API... where your request could look like this: <request>abc=123&def=456</request>.
Even better, note the & is not an & - you can't even use a standard XML parser to interpret the result.
Also: APIs that have a mix of strongly and eventually consistent calls and no documentation about which is which. Or, an API that was at one point strongly consistent turns eventually consistent without warning.
I also once worked with an API where you had to send the data in POST format - abc=123&def=456. After much pressure from their customers, they finally relented and added an XML version of their API... where your request could look like this: <request>abc=123&def=456</request>.
Even better, note the & is not an & - you can't even use a standard XML parser to interpret the result.
Also: APIs that have a mix of strongly and eventually consistent calls and no documentation about which is which. Or, an API that was at one point strongly consistent turns eventually consistent without warning.