All the LMs landed a few degrees off-level. Apollo 11 was closest to level at about 4 degrees. The design limit for LM ascent stage lift-off is variously stated as 12 or 15 degrees.
Apollo 15 was tilted somewhere around 11 degrees (6.9 degree pitch-up and 8.6 degrees left roll).
It's unclear me to what the rationale for the design limit is; the ascent stage has a very good thrust-to-weight ratio and attitude control, and could launch from even a very steep tilt; in fact, if the LM started to fall over after touchdown, it was possible to do an ascent-stage abort liftoff with the LM tilted as much as 60 degrees, or even further with an emergency manual control mode.
Upon landing, with the descent stage fuel tanks empty and the ascent stage full, the LM's center of gravity is fairly high. The stationary tip-over limit for the LM is a little over 40 degrees (the point on the diagram where the neutral stability line crosses the vertical axis).
I imagine that if one of the landings had come down with an unacceptably steep tilt, the astronauts would have been instructed to dig surface material out from under the high legs until it was more level. Mission control would probably also press Grumman for a detailed risk assessment. If they ran out of consumables time before leveling the LM, they'd obviously have attempted launch anyway; they'd have nothing to lose.
I've skimmed the Apollo 15 annotated transcripts for references to the tilt. The flight director was notified of the tilt (seen via telemetry) a couple minutes after landing, around 104:45. Around 104:50 is this note:
In Houston, the Flight Director asks if the spacecraft tilt is acceptable and wants to review the relevant pre-flight assessments of the margins of safety.
The tilt had some housekeeping implications; they had to repoint their S-band antenna and sight on different stars than originally planned to realign their inertial platform.
At 116:33:39 there's discussion about the impact of the LM tilt on rover deployment -- previously tested and no problem at 15 degrees. Around 120:26 they have some minor tilt-related difficulty deploying the rover. Later, there's a water leak in the cabin that goes unnoticed for a while because the tilt is causing it to puddle in one corner.
Apart from that, throughout the surface stay and departure, there are no other mentions of it, so I have to assume the FD was reassured that the tilt was within spec. In particular, stage separation and ascent engine ignition are two separate manual operations, and there's no discussion of the timing between them, i.e. they didn't seem worried about the ascent stage sliding in between.