You can over grease especially when it comes to over greasing bearings.
And if you over grease an electric motor bearing it can cause the grease to get into the electric motor itself and cause smoking and even failure of the electric motor.
For joints and not bearings it's still possible to over grease and can lead to the seals popping out or tearing and cause the excess grease to leak out although it's not as bad as over greasing wheel bearings and other bearings.
Putting too much grease in a bearing can shorten the bearings lifespan and lead to churning.
Churning of the grease is when the excess grease causes the bearings elements to work harder and generate excessive heat.
This excessive heat generation degrades the grease and causes premature bearing failure.
Too much much added to the bearing can also cause grease to push past the seal of the bearing as well and could damage the seal.
When greasing a bearing you want to add enough grease to the bearing so that the grease pushes out from the bearing and then no more is needed.
Packing bearings with too much grease can also over pressurize the grease when it's hot which is also not good.
One of the most common mistakes when greasing bearings is adding too much grease to the bearings or too little grease to the bearings.
Too much grease in the bearings builds up and eventually causes increased pressure and increased friction on the bearing and surfaces which leads to bearing failure and overheating of the bearings.
Too little grease in the bearings also shortens the life of the bearings as the will wear out faster as a result of increased heat and friction.
When a bearing has enough grease, it should have enough grease to fill the bearing space, without causing any excessive churning or overheating.
You can tell by listening for a change in bearing sound as the bearing should get quieter when it has enough grease and there should be some resistance on the grease gun handle.
Or if greasing bearings through a grease fitting some grease should come out of the grease fitting when it has enough grease.