Sean Dyche claimed Watford were "not good enough" following their 2-1 defeat against Reading.
Seven of the previous 11 meetings between the two sides had ended in draws and that sequence looked like extending to eight in 12 until a late Royals corner was flicked on and the substitute pulled off his man at the back post to hook the ball past Scott Loach.
Earlier, Jimmy Kebe had headed in a Hal Robson-Kanu cross to get the visitors back on terms before the interval after Shaun Cummings had diverted a Troy Deeney cross past his own goalkeeper.
Dyche said: "I didn't think our performance level was good enough for us. We've performed very well of late but there were too many individual performances that weren't quite right.
"
You can carry maybe one or two but if it goes to three and four then that affects the team and all of a sudden the team looked a bit disjointed.
"The second half was a bit more even, the first half they were the better side I felt, so eventually you get caught out if your performance levels are not high enough."