$ git grep ProtocolError
httputil/persist.go:    ErrPersistEOF = &http.ProtocolError{ErrorString: "persistent connection closed"}
httputil/persist.go:    ErrClosed     = &http.ProtocolError{ErrorString: "connection closed by user"}
httputil/persist.go:    ErrPipeline   = &http.ProtocolError{ErrorString: "pipeline error"}
request.go:type ProtocolError struct {
request.go:func (err *ProtocolError) Error() string { return err.ErrorString }
request.go:     ErrHeaderTooLong        = &ProtocolError{"header too long"}
request.go:     ErrShortBody            = &ProtocolError{"entity body too short"}
request.go:     ErrNotSupported         = &ProtocolError{"feature not supported"}
request.go:     ErrUnexpectedTrailer    = &ProtocolError{"trailer header without chunked transfer encoding"}
request.go:     ErrMissingContentLength = &ProtocolError{"missing ContentLength in HEAD response"}
request.go:     ErrNotMultipart         = &ProtocolError{"request Content-Type isn't multipart/form-data"}
request.go:     ErrMissingBoundary      = &ProtocolError{"no multipart boundary param in Content-Type"}