Cobra has pretty much nailed it.
Making the odd mistake is human, repeating them is when you need to start worrying
As several people have said do not keep gate loaded when grouping, you can put that valuable spell slot to better use and if you think like most Clerics, you would expect to be there trying to keep your people alive to the bitter end, evacs are the Druid/Wizzy's problem to worry about.
I think Cobra made a very important point about routine.
Before the spell set load and save facility was introduced, there was many a time I would sit staring at my spell book trying to remember the you beaut way I used to have my spells loaded for "x" situation.
Now I have them all nicely saved as sets and with a push of a button I get exactly the right set up each time I need it, with out thinking. (Except when I have levelled and forgotten to replace the old spell in the set
)
One of the things I have noticed about being a cleric, compared to the other classes I have played in a serious way, is that it often demands a very high degree of sustained concetration. Druid healing in the Planes is no picknic, but there is something about Clericking that I can't quite put my finger on. Anyway I find it to be almost as intense as playing a Chanter and at times you do get mentally fatigued. These are the times when you find yourself doing inexplicable things like hitting alt 3 instead of 3.
Having recognised this, now I try to look for the signs during a long session and shake myself up a bit when it seems to be setting in.
By the way, I know this comes under the heading of personal preference, but I do not include the cast command in my heal hot button, just the warning to stay in range and stand. I much prefer to hit he heal cast directly, either with alt 3 ( yes that is my CH too
) or mouse click. I find that I often want to control the time between broadcasting the message and casting, like when I need to chase the darned tank down the hall way, lol.