https://play.golang.org/p/u7Dfo4UnxZ0

assert.Contains(t, "error string", errors.New("error"))

Error Trace:	prog.go:25
Error:      	"error string" does not contain "error"
Test:       	TestCaseA

Whereas this is what's needed to pass:

-assert.Contains(t, "error string", errors.New("error"))
+assert.Contains(t, "error string", errors.New("error").Error())

I would prefer if the failure message explained that it was an error type, otherwise it looks like a string does not contain another string: I've fallen foul to this in the past, and just spent 5-10mins again not realising that I needed to pass a string type as the 3rd argument.

It would be really nice to have the type printed here when the value isn't a Stringer =)