I am building a simple reverse-polarity circuit using two relays. Will it create an endless loop?
EDIT:
You have changed the schematic. Yes, in your schematic, at any point, out of the box or &quot;aged&quot; you can potentially be shorting the battery at every single switch over. It may be unlikely if the relays are exactly the same, but it is a very real risk, at any point in time.
This is why the schematic you posted earlier, with the relays connected the other way around, with the motor at the centretaps, is the way it is. Because the designer of that circuit had thought ahead about the implications of production margins. Or it was by accident, but the result is very much the same: Their design is safe, because only the motor can ever be shorted, while your new schematic isn't safe, because the supply can be shorted.
To make sure people coming by later understand what we're all on about, I shall redraw your originally posted schematic, followed by my original answer:



simulate this circuit &ndash; Schematic created using CircuitLab
If you carefully trace all the possible connections with these relays you will see that your battery cannot be shorted.
Your motor can be shorted, but that's the main point, that's how you turn it off (and also put the brake on). If both relays are up, as drawn, the motor is shorted to the battery +, while the battery - is disconnected. If both are down, the motor is shorted to battery -, while battery + is floating. If they are in different positions the motor rotates left or right.