Season 4, episode 8 "Opie's ill gotten gain".
Early in the show Andy and Barney are sitting at the desk in the Court house, Barney is claiming that he knows the preamble to the US constitution, and Andy is leading him by reading it line by line, and of course Barney is muffing it up.

In the distant scenes that show both men facing one another, Barney's hair is perfectly combed. BUT, they cut to closeups of Barney's head several times, and each time they do, Barney has a "Larry Fine" style hairdo (without the pattern baldness).
Obviously the close ups were shot separately, in a different session

Yup that’s a bad continuity problem that almost takes away from the hilarity of the bit.

Yup that’s a bad continuity problem that almost takes away from the hilarity of the bit.

It actually did for me a bit. After the first close up where Barney is mussing up his own hair, I was in stitches. But then when they cut away to a distant shot, showing both characters, his hair is perfectly normal once again. It forced me to abandon the "funny bone" of the moment just to sort out the inconsistency. Wondering if the mistake was on me.

Then, by the time they returned to the close up view again, I could tell that the mistake was on them, .....but by that time the humor was gone.

The lapse in continuity harpooned the timing of the humor, IMO.

I see what you mean but other scenes where there is no continuity trouble saves the show. Andy and Barney were almost as funny as the early Stooges. Almost.

Because the show was filmed and not taped, the close-up (or single actor) shots were always done separately and sometime even on different days. This example is an extreme one with his hair, but you can see differences in many episodes and scenes when they go back and forth between the wide shots and the close-ups. Very often there are subtle differences.

On other taped shows (not TAGS), it can happen too. Many taped shows did two tapings and the final aired episodes contain the best shots between the two tapings. You can often see small differences there as well.