And some very high speed motors might burst/explode if run at twice max rated speed (OK, this is far-fetched)
Bearing wear could be excessive due to much higher vibration levels.
Some motors with permanent magnets could heat enough to approach the Curie temperature, causing the magnetisation to start falling, which will cause the speed to increase (so more friction, more current, more heat, viscious circle).
A motor with integral fan will of course have a much higher flow of cooling air at higher speeds, mitigating the risk of excessive temperature, but not addressing mechanical issues.
Overall useful lifetime of motor could be drastically reduced.