If a miner A creates a block containing a transaction A and a miner B creates a block NOT containing the transaction A about at the same time, then some of the other miners will first receive the block of the miner A and the others will receive the block of the miner B. (Fork)
If the chain of the miner B grows faster than the chain of the miner A, then the transaction A will never be accepted (like it never happened). So, how the user A who created the transaction A can be sure that the transaction A was accepted/verified by the blockchain?
Also, is there a possibility that the transaction A is verified by the blockchain at some point and then in the future another subchain (from a previous fork) not containing the transaction A will grow faster, thus rejecting transaction A ?
Are there any time checkpoints created that all the miners should agree upon ?