Watford were denied their first home win of the season after George Friend's injury-time strike earned Doncaster a point from a 2-2 draw at Vicarage Road.
Doncaster made a good start and took the lead in the 32nd minute when John Oster played in Simon Gillett and he finished well for his first goal in nearly four years.
Hornets boss Malky Mackay brought Marvin Sordell on 10 minutes after the break and the change paid dividends when he headed home Don Cowie's cross with 63 minutes gone.
Seven minutes later the impressive Sordell got on the end of goalkeeper Scott Loach's long punt upfield and fired past Neil Sullivan to put Watford in front.
It looked like that would be enough to get Watford the three points but two minutes into injury-time Martin Woods' corner found Friend and he stabbed in a dramatic equaliser.