It should be painless
If the process is done right, cardiac arrest should happen soon enough and the condemned should not experience pain. However, there are many debates whether lethal injection actually works the way it was designed to.
Burning sensation could be felt
Some people argue that asphyxiation, a severe burning sensation, could happen if the procedure was not done correctly or if the dose wasn’t in the right amount. (See How does burning to death feel like?)
Massive muscle cramping might happen
Without adequate anaesthesia, massive muscle cramping might happen thus causing a lot of pain to the condemned. Debates are still going on whether this process can be painful or not.
Some people might be fully aware
According to one argument, the difference in the dose administrated might make some people fully aware of the process. Those people will fail to report any pain because the muscle relaxant added to the mixture fully paralyzes them.
Inexperienced people could make it worse
Because doctors aren’t usually allowed to help in such executions, inexperienced people usually do the drug administration part. This could result in wrong doses, which could make the condemned feel pain. (See What does it feel like to die?)
A paralytic arrests breathing
As a part of the normal procedure, a paralytic that arrests breathing is introduced after the person has been sedated. Without proper sedation, pain might be experienced.
In all cases, the strong muscle relaxant given subjects the condemned to complete paralysis. The condemned would thus fail to report pain if it was experienced.
Death by suffocation
If the proper dose of the sedative agent wasn’t administered then the person might be conscious when the substance that stops breathing is introduced.