What are signs of incompatible memory?

Hello.. I have had a look at previous Q & A and am satisfied that 1.5 GB ram is fine in the eMac however i would like to know what the symptoms would be if there was memory incompatibility.

There is a long story , here is the short version.... Basically things went into a frenzy, wouldn't even shut down. The (additional) memory wasn't put in at my place and the computer also had updates done by the other person, when it came back it was unbelievable.. and worrying... my son refitted the 2 sticks that were in it, still frenzy, thought it was wrecked, looked up book, did safe boot. after angonising minutes everything came back.

Gathered courage and we re installed the (new) 1gb alongside incumbent 512 and now things seem ok.

I don't believe the ram is /was faulty so something else went on.

The sticks that are in the machine are 512gb DDR SDRam PC3200U 30440 ( this was one of a matched pair)

1 gb DDR SDRam PC 3200U 30330 ( this is the new one)

I felt i bought the 1gb carefully to be compatible. I do see there is a difference in the last number. Thanks.