Can I Freeze It While I Work On A Bar?

You can attach a couple of screenshots?
Before and after the appearance of of the new measure?
You did not use the Ins key, right? (see Preferences / shortcuts)
What operating system do you use?

I'm almost illiterate (musically and computer science), but with a little 'practice, this program gives great satisfaction. It just have a little patience, and passion;-)
I used MuseScore even on older PCs (also from CD or pendrive), it works and is stable.

Copy and paste is indeed a powerful technique - not sure what you mean in asking if it's easier to do that than to "fix". Depends on what you are trying to fix, what's wrong with it, etc. But copy and paste is the usual method of nothing duplicating passages, but also moving them earlier or later in time should you have made a mistake in initial entry that causes everything to be off by a note or whatever.

MuseScore is quite stable. But it does take a while to get to used to the input method for any notation program. I am quite sure the problems you are experiencing stem from your inexperience - trial and error does not tend to work well as an input method. Definitely read the whole section on Note Entry in the Handbook and watch all the tutorial videos on the main page. They help avoid some of the more common mistakes. And then, if you can be more specific about exactly what you are trying to do, people can help better.

Good luck!

Computer power:
I discovered MuseScore back when I was happy to simply print blank manuscript paper. I was running an Athlon Xp 1700+ (1467 MHz) with 512MB ram. Of course that was when, in MuseScore, one could choose *any* instrument for playback - so long as it was piano.
As development proceeded to the point of being able to change soundfonts, some people may have experienced delays during playback if they had a slow computer and a large soundfont.

Also, my video card was a 32MB GeForce2 MX200 - I never had display problems. I mean, after all, MuseScore is not as demanding as a video intensive game. Having said that, a few forum posters have reported computer slowdowns with very large orchestral scores.
If, as you say, ' my needs are small' - this prog is definitely a keeper.

Robust / finicky... rock solid / twitchy:
First off, I cannot stess the importance of the handbook and tutorials, especially if you have used another notation program which enters notes differently. Some forum members have complained about MuseScore's 'tyranny' when entering notes, and somehow that it corrupts subsequent notes/measures - by adding rests/ties. Suffice it to ask that if one adds a half note (minim) to the very last beat of a measure, where should the second beat of that half note (minim) go?
Of course, to the next measure (tied across the barline), replacing any note that might already be there; instead of moving the entire score (notes, beams, chord symbols, dynamic markings, etc.) one beat over to 'make room'.

Conversely, if one overwrites a half note (minim) with a quarter note (crotchet), MuseScore's behavior is to accomodate the quarter note (crotchet), and enter a rest where the 'extra time' now occurs - instead of filling it up by moving the entire score (in the opposite direction this time). That's why copy/paste is a good thing to know about.

Therefore, one needs to develop a good understanding... which is not to say that there are never any bugs / glitches / finicky behavior:
When that little sprite (that floats around the page when one is in note entry mode) first appeared in a MuseScore revision, I would invariably make an errant click somewhere on the score and create some crazy note with a bazillion ledger lines -- yikes! I was perfectly happy with the way things were - why mess it up? Then it hit me that this program is constantly under development.
So, definitely a keeper.

Another reason (the forums):
Besides the dedication of the developers, these forums are a great source of information - not only with respect to MuseScore - but as a source of learning about music and the global community in general. (I enjoy reading some of the 'Google translations' - not to mention the occasional 'heated' discussion.)
Also, the turnaround time for questions in these forums is quick, relative to other support sites.

OK, I'm taking way too much time (I see other people have posted here).

So... concerning your attached score:
If you are copying 'Smoke Gets in Your Eyes' from an existing sheet, it's probably best to enter the key signature first - so in this case, MuseScore will flat the 'B' notes - i.e. pitches (as directed by use of the key signature).

Entering notes with no key signature (or more precisely, in the key of C) causes the B's to sound and appear natural (have a listen to your score).
One would have to enter a flat for each B to get the melody to 'sound right' (with no sharps/flats in the key signature).
Adding a key signature *after* note entry does not cause MuseScore to change previously entered pitches - the pitches/notes will *sound* the same as was entered (even though they may *look* different with accidental(s) added or removed, as a different key signature is applied to an existing score).
For example, try dragging a G major key signature (one sharp) to your score - any F will show a natural sign, still sounding the same as when entered.

If one uses the C major key signature, and must enter all B flats to make the tune 'sound right' - consider it a clue to score in F major (one flat: B). So, dragging the F major key signature to that score will keep the pitches of the notes the same - so the melody still *sounds* the same - but will eliminate all the flat signs on the B notes.

Hope you can follow this, as it assumes you are *not* using the little known 'Alternate input method' which is found in note entry preferences (off by default).

Also, here is a (sometimes rowdy) discussion regarding key sigs and note entry:
http://musescore.org/en/node/22887

I must go... Regards.