[morzh]

The equivalent of one short turn is basically the same as that of a shorted load.
If it is the primary's turn, then the load is the loss resistance of that turn, which is just a smidge higher than would be the loss of a turn of the secondary, due to large output winding's wire's diameter.
If supplied with the signal, it will induce current in it and cause heating.
If not (just the DC bias is applied, but no AC component is present), then the transformer will not heat up anymore than as if it were OK. That is the resistive losses in the primary.

The shorted load (if it is the secondary) will cause a total short, as it is close to a milliOhm or so. If in the primary, it could be close to 0.05 - 0.1 Ohms and is comparable to early speakers' voice coil resistance, so there will be a little (very little) signal coming out. Plus heating/smoking etc, if the signal is strong.